macOS: always set `MACOSX_DEPLOYMENT_TARGET` (#28797)

* core: Make platform environment an instance not class method

In preparation for accessing data constructed in __init__.

* macos: set consistent macosx deployment target

This should silence numerous warnings from mixed gcc/macos toolchains.

* perl: prevent too-new deployment target version

```
*** Unexpected MACOSX_DEPLOYMENT_TARGET=11
***
*** Please either set it to a valid macOS version number (e.g., 10.15) or to empty.
```

* Stylin'

* Add deployment target overrides to failing autoconf packages

* Move configure workaround to base autoconf package

This reverts commit 3c119eaf8b4fb37c943d503beacf5ad2aa513d4c.

* Stylin'

* macos: add utility functions for SDK

These aren't yet used but should probably be added to spack debug
report.

5 files changed, 75 insertions, 6 deletions

diff --git a/lib/spack/spack/build_systems/autotools.py b/lib/spack/spack/build_systems/autotools.py
index e03870d0f9..46a3ccddf7 100644
--- a/lib/spack/spack/build_systems/autotools.py
+++ b/lib/spack/spack/build_systems/autotools.py
@@ -15,8 +15,10 @@ from llnl.util.filesystem import force_remove, working_dir
 
 from spack.build_environment import InstallError
 from spack.directives import depends_on
+from spack.operating_systems.mac_os import macos_version
 from spack.package import PackageBase, run_after, run_before
 from spack.util.executable import Executable
+from spack.version import Version
 
 
 class AutotoolsPackage(PackageBase):
@@ -415,6 +417,13 @@ To resolve this problem, please try the following:
         with working_dir(self.build_directory, create=True):
             inspect.getmodule(self).configure(*options)
 
+    def setup_build_environment(self, env):
+        if (self.spec.platform == 'darwin'
+                and macos_version() >= Version('11')):
+            # Many configure files rely on matching '10.*' for macOS version
+            # detection and fail to add flags if it shows as version 11.
+            env.set('MACOSX_DEPLOYMENT_TARGET', '10.16')
+
     def build(self, spec, prefix):
         """Makes the build targets specified by
         :py:attr:``~.AutotoolsPackage.build_targets``
diff --git a/lib/spack/spack/operating_systems/mac_os.py b/lib/spack/spack/operating_systems/mac_os.py
index 226bfccca5..cb430449f2 100644
--- a/lib/spack/spack/operating_systems/mac_os.py
+++ b/lib/spack/spack/operating_systems/mac_os.py
@@ -4,6 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import platform as py_platform
+import re
 
 from spack.util.executable import Executable
 from spack.version import Version
@@ -18,11 +19,49 @@ def macos_version():
     return Version(py_platform.mac_ver()[0])
 
 
+def macos_cltools_version():
+    """Find the last installed version of the CommandLineTools.
+
+    The CLT version might only affect the build if it's selected as the macOS
+    SDK path.
+    """
+    pkgutil = Executable('pkgutil')
+    output = pkgutil('--pkg-info=com.apple.pkg.CLTools_Executables',
+                     output=str, fail_on_error=False)
+    match = re.search(r'version:\s*([0-9.]+)', output)
+    if match:
+        return Version(match.group(1))
+
+    # No CLTools installed by package manager: try Xcode
+    output = pkgutil('--pkg-info=com.apple.pkg.Xcode',
+                     output=str, fail_on_error=False)
+    match = re.search(r'version:\s*([0-9.]+)', output)
+    if match:
+        return Version(match.group(1))
+
+    return None
+
+
 def macos_sdk_path():
-    """Return SDK path
+    """Return path to the active macOS SDK.
+    """
+    xcrun = Executable('xcrun')
+    return xcrun('--show-sdk-path', output=str).rstrip()
+
+
+def macos_sdk_version():
+    """Return the version of the active macOS SDK.
+
+    The SDK version usually corresponds to the installed Xcode version and can
+    affect how some packages (especially those that use the GUI) can fail. This
+    information should somehow be embedded into the future "compilers are
+    dependencies" feature.
+
+    The macOS deployment target cannot be greater than the SDK version, but
+    usually it can be at least a few versions less.
     """
     xcrun = Executable('xcrun')
-    return xcrun('--show-sdk-path', output=str, error=str).rstrip()
+    return Version(xcrun('--show-sdk-version', output=str).rstrip())
 
 
 class MacOs(OperatingSystem):
diff --git a/lib/spack/spack/platforms/_platform.py b/lib/spack/spack/platforms/_platform.py
index 61845eb430..b68b7f1299 100644
--- a/lib/spack/spack/platforms/_platform.py
+++ b/lib/spack/spack/platforms/_platform.py
@@ -104,11 +104,11 @@ class Platform(object):
 
         return self.operating_sys.get(name, None)
 
-    @classmethod
-    def setup_platform_environment(cls, pkg, env):
+    def setup_platform_environment(self, pkg, env):
         """Subclass can override this method if it requires any
         platform-specific build environment modifications.
         """
+        pass
 
     @classmethod
     def detect(cls):
diff --git a/lib/spack/spack/platforms/cray.py b/lib/spack/spack/platforms/cray.py
index 133c29940e..23724dffbe 100644
--- a/lib/spack/spack/platforms/cray.py
+++ b/lib/spack/spack/platforms/cray.py
@@ -84,8 +84,7 @@ class Cray(Platform):
         if self.front_os != self.back_os:
             self.add_operating_system(self.front_os, front_distro)
 
-    @classmethod
-    def setup_platform_environment(cls, pkg, env):
+    def setup_platform_environment(self, pkg, env):
         """ Change the linker to default dynamic to be more
             similar to linux/standard linker behavior
         """
diff --git a/lib/spack/spack/platforms/darwin.py b/lib/spack/spack/platforms/darwin.py
index 7f38cad42e..b4b542cb98 100644
--- a/lib/spack/spack/platforms/darwin.py
+++ b/lib/spack/spack/platforms/darwin.py
@@ -39,3 +39,25 @@ class Darwin(Platform):
     @classmethod
     def detect(cls):
         return 'darwin' in platform.system().lower()
+
+    def setup_platform_environment(self, pkg, env):
+        """Specify deployment target based on target OS version.
+
+        The ``MACOSX_DEPLOYMENT_TARGET`` environment variable provides a
+        default ``-mmacosx-version-min`` argument for GCC and Clang compilers,
+        as well as the default value of ``CMAKE_OSX_DEPLOYMENT_TARGET`` for
+        CMake-based build systems. The default value for the deployment target
+        is usually the major version (11, 10.16, ...) for CMake and Clang, but
+        some versions of GCC specify a minor component as well (11.3), leading
+        to numerous link warnings about inconsistent or incompatible target
+        versions. Setting the environment variable ensures consistent versions
+        for an install toolchain target, even when the host macOS version
+        changes.
+
+        TODO: it may be necessary to add SYSTEM_VERSION_COMPAT for older
+        versions of the macosx developer tools; see
+        https://github.com/spack/spack/pull/26290 for discussion.
+        """
+
+        os = self.operating_sys[pkg.spec.os]
+        env.set('MACOSX_DEPLOYMENT_TARGET', str(os.version))