apple-clang: add correct path to compiler wrappers (#21662)

Follow-up to #17110

### Before
```bash
CC=/Users/Adam/spack/lib/spack/env/clang/clang; export CC
SPACK_CC=/usr/bin/clang; export SPACK_CC
PATH=...:/Users/Adam/spack/lib/spack/env/apple-clang:/Users/Adam/spack/lib/spack/env/case-insensitive:/Users/Adam/spack/lib/spack/env:...; export PATH
```

### After
```bash
CC=/Users/Adam/spack/lib/spack/env/clang/clang; export CC
SPACK_CC=/usr/bin/clang; export SPACK_CC
PATH=...:/Users/Adam/spack/lib/spack/env/clang:/Users/Adam/spack/lib/spack/env/case-insensitive:/Users/Adam/spack/lib/spack/env:...; export PATH
```

`CC` and `SPACK_CC` were being set correctly, but `PATH` was using the name of the compiler `apple-clang` instead of `clang`. For most packages, since `CC` was set correctly, nothing broke. But for packages using `Makefiles` that set `CC` based on `which clang`, it was using the system compilers instead of the compiler wrappers. Discovered when working on `py-xgboost@0.90`.

An alternative fix would be to copy the symlinks in `env/clang` to `env/apple-clang`. Let me know if you think there's a better way to do this, or to test this.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/spack/spack/build_environment.py b/lib/spack/spack/build_environment.py
index d023365bba..f7fd76d82a 100644
--- a/lib/spack/spack/build_environment.py
+++ b/lib/spack/spack/build_environment.py
@@ -417,7 +417,7 @@ def set_build_environment_variables(pkg, env, dirty):
     # directory.  Add that to the path too.
     env_paths = []
     compiler_specific = os.path.join(
-        spack.paths.build_env_path, pkg.compiler.name)
+        spack.paths.build_env_path, os.path.dirname(pkg.compiler.link_paths['cc']))
     for item in [spack.paths.build_env_path, compiler_specific]:
         env_paths.append(item)
         ci = os.path.join(item, 'case-insensitive')