[mutter/wip/lantw/dont-hardcode-usr-bin-python3] build: Don't hardcode the path of python3

[Ting-Wei Lan <lantw src gnome org>]

commit 740c2298c6a649f65378b1d7adcd9ebbe1328c99
Author: Ting-Wei Lan <lantw src gnome org>
Date:   Sun Nov 18 14:16:24 2018 +0800

    build: Don't hardcode the path of python3
    
    Python is not guaranteed to be installed in /usr/bin. This is especially
    true for *BSD systems which don't consider Python as an integral part of
    their systems.

 src/backends/native/gen-default-modes.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/backends/native/gen-default-modes.py b/src/backends/native/gen-default-modes.py
index 304cd0303..aeccc0d28 100755
--- a/src/backends/native/gen-default-modes.py
+++ b/src/backends/native/gen-default-modes.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 
 # Copyright (C) 2016 Red Hat Inc.
 #