OpenGL: add spack external find support (#18003)

var/spack/repos/builtin/packages/opengl/package.py

@@ -3,10 +3,9 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
+import re
 import sys
 
-from spack import *
-
 
 class Opengl(Package):
     """Placeholder for external OpenGL libraries from hardware vendors"""
@@ -35,6 +34,14 @@ class Opengl(Package):
     if sys.platform != 'darwin':
         provides('glx@1.4')
 
+    executables = ['^glxinfo$']
+
+    @classmethod
+    def determine_version(cls, exe):
+        output = Executable(exe)(output=str, error=str)
+        match = re.search(r'OpenGL version string: (\S+)', output)
+        return match.group(1) if match else None
+
     # Override the fetcher method to throw a useful error message;
     # fixes GitHub issue (#7061) in which this package threw a
     # generic, uninformative error during the `fetch` step,