gcc: support without command line tools (#11040)

lib/spack/spack/operating_systems/mac_os.py

@@ -7,6 +7,7 @@
 
 from spack.architecture import OperatingSystem
 from spack.version import Version
+from spack.util.executable import Executable
 
 
 # FIXME: store versions inside OperatingSystem as a Version instead of string
@@ -16,6 +17,13 @@ def macos_version():
     return Version('.'.join(py_platform.mac_ver()[0].split('.')[:2]))
 
 
+def macos_sdk_path():
+    """Return SDK path
+    """
+    xcrun = Executable('xcrun')
+    return xcrun('--show-sdk-path', output=str, error=str).rstrip()
+
+
 class MacOs(OperatingSystem):
     """This class represents the macOS operating system. This will be
     auto detected using the python platform.mac_ver. The macOS