swig: support external find (#31990)

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

@@ -4,6 +4,7 @@
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
 import os
+import re
 
 from spack.package import *
 
@@ -22,7 +23,9 @@ class Swig(AutotoolsPackage, SourceforgePackage):
     sourceforge_mirror_path = "swig/swig-3.0.12.tar.gz"
     maintainers = ["sethrj"]
 
-    tags = ["e4s"]
+    tags = ["e4s", "build-tools"]
+
+    executables = ["^swig$"]
 
     version("master", git="https://github.com/swig/swig.git")
     version(
@@ -66,6 +69,12 @@ class Swig(AutotoolsPackage, SourceforgePackage):
 
     conflicts("%nvhpc", when="@:4.0.2")
 
+    @classmethod
+    def determine_version(cls, exe):
+        output = Executable(exe)("-version", output=str, error=str)
+        match = re.search(r"SWIG\s+Version\s+(\S+)", output)
+        return match.group(1) if match else None
+
     @run_after("install")
     def create_symlink(self):
         # CMake compatibility: see https://github.com/spack/spack/pull/6240