build: Add script to detect files in XDG_DATA_DIRS

meson doesn't have support for doing this, so use Python and just the
stdlib to find files under XDG_DATA_DIRS and XDG_DATA_HOME.

build-aux/meson/find_xdg_file.py

@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+
+import os
+import sys
+
+def usage():
+    print('Usage:')
+    print('find_xdg_file.py FILENAME')
+    print('')
+    print('Looks for FILENAME in the XDG data directories and returns if path if found')
+
+if len(sys.argv) != 2:
+    usage()
+    sys.exit(1)
+
+filename = sys.argv[1]
+
+data_home = os.getenv('XDG_DATA_HOME')
+if not data_home or data_home == '':
+    data_home = os.path.join(os.path.expanduser("~"), "local", "share")
+
+data_dirs_str = os.getenv('XDG_DATA_DIRS')
+if not data_dirs_str or data_dirs_str == '':
+    data_dirs_str = '/usr/local/share/:/usr/share/'
+
+dirs = []
+dirs += [ data_home ]
+for _dir in data_dirs_str.split(':'):
+    dirs += [ _dir ]
+
+for _dir in dirs:
+    full_path = os.path.join(_dir, filename)
+    if os.path.exists(full_path):
+        print(full_path)
+        sys.exit(0)
+
+print(f"'{filename}' not found in XDG data directories")
+sys.exit(1)