GDAL: add spack external find support (#18004)

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

@@ -165,6 +165,12 @@ class Gdal(AutotoolsPackage):
 
     conflicts('+mdb', when='~java', msg='MDB driver requires Java')
 
+    executables = ['^gdal-config$']
+
+    @classmethod
+    def determine_version(cls, exe):
+        return Executable(exe)('--version', output=str, error=str).rstrip()
+
     def setup_build_environment(self, env):
         # Needed to install Python bindings to GDAL installation
         # prefix instead of Python installation prefix.