ninja: support spack find external (#19616)

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

@@ -2,8 +2,7 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-from spack import *
+import re
 
 
 class Ninja(Package):
@@ -16,6 +15,8 @@ class Ninja(Package):
     url      = "https://github.com/ninja-build/ninja/archive/v1.7.2.tar.gz"
     git      = "https://github.com/ninja-build/ninja.git"
 
+    executables = ['^ninja$']
+
     version('kitware', branch='features-for-fortran', git='https://github.com/Kitware/ninja.git')
     version('master', branch='master')
     version('1.10.1', sha256='a6b6f7ac360d4aabd54e299cc1d8fa7b234cd81b9401693da21221c62569a23e')
@@ -29,6 +30,11 @@ class Ninja(Package):
 
     phases = ['configure', 'install']
 
+    @classmethod
+    def determine_version(cls, exe):
+        output = Executable(exe)('--version', output=str, error=str)
+        return output.strip()
+
     def configure(self, spec, prefix):
         python('configure.py', '--bootstrap')