XorgPackage, glx: conflicts windows and darwin (#50216)

* XorgPackage: conflicts windows and darwin

* xorg: import conflicts

* xorg: fix style

* glx: conflicts windows and darwin

lib/spack/spack/build_systems/xorg.py

@@ -6,12 +6,17 @@
 
 import spack.package_base
 import spack.util.url
+from spack.directives import conflicts
 
 
 class XorgPackage(spack.package_base.PackageBase):
     """Mixin that takes care of setting url and mirrors for x.org
     packages."""
 
+    # x.org packages are not supported on windows or darwin
+    conflicts("platform=windows")
+    conflicts("platform=darwin")
+
     #: Path of the package in a x.org mirror
     xorg_mirror_path: Optional[str] = None