py-cffi: add compiler flags to fix build with clang (#29679)

* py-cffi: add compiler flags to fix build with clang

For %clang@13.0.1, this avoids the
```
clang-13: warning: optimization flag '-ffat-lto-objects' is not supported [-Wignored-optimization-argument]
```
warning being turned into an error, and fixes this link error:
```
build/temp.linux-x86_64-3.10/c/_cffi_backend.o: file not recognized: file format not recognized
```

* style

1 files changed, 10 insertions, 0 deletions

diff --git a/var/spack/repos/builtin/packages/py-cffi/package.py b/var/spack/repos/builtin/packages/py-cffi/package.py
index be768f6286..711e601635 100644
--- a/var/spack/repos/builtin/packages/py-cffi/package.py
+++ b/var/spack/repos/builtin/packages/py-cffi/package.py
@@ -28,6 +28,16 @@ class PyCffi(PythonPackage):
     depends_on('py-pycparser', type=('build', 'run'))
     depends_on('libffi')
 
+    def flag_handler(self, name, flags):
+        if self.spec.satisfies('%clang@13:'):
+            if name in ['cflags', 'cxxflags', 'cppflags']:
+                flags.append('-Wno-error=ignored-optimization-argument')
+                return (flags, None, None)
+            if name == 'ldflags':
+                flags.append('-flto')
+                return (flags, None, None)
+        return (flags, None, None)
+
     def setup_build_environment(self, env):
         # This sets the compiler (and flags) that distutils will use
         # to create the final shared library.  It will use the