[gimp/gtk3-port-meson] Fix: disable custom target when python not found

[Félix Piédallu <fpiedallu src gnome org>]

commit 0a53c22033b8a5e15b844b9d10d3dcf3a72f9a57
Author: Félix Piédallu <felix piedallu me>
Date:   Mon Feb 12 17:40:34 2018 +0100

    Fix: disable custom target when python not found

 meson.build |   23 ++++++++++++-----------
 1 files changed, 12 insertions(+), 11 deletions(-)
---
diff --git a/meson.build b/meson.build
index 045b8dd..df9028d 100644
--- a/meson.build
+++ b/meson.build
@@ -1084,17 +1084,18 @@ if xmllint.found()
 endif
 
 # TODO this python script is waiting for autotools directory/files structure
-custom_target('check-defs',
-  input : [ ],
-  output: [ 'check-defs', ],
-  command: [
-    python2.path(),
-    join_paths(meson.source_root(), 'tools','defcheck.py'),
-    meson.source_root(),
-  ],
-  build_by_default: false,
-)
-
+if have_python
+  custom_target('check-defs',
+    input : [ ],
+    output: [ 'check-defs', ],
+    command: [
+      python2.path(),
+      join_paths(meson.source_root(), 'tools','defcheck.py'),
+      meson.source_root(),
+    ],
+    build_by_default: false,
+  )
+endif
 
 custom_target('Changelog',
   input : [ ],