[jhbuild] environment: Fix KeyError when CONFIG_SITE is unset.

[Hans Petter <hansp src gnome org>]

commit 1e81db425f76eb0fad66e641c47f28faf6683d0f
Author: Hans Petter Jansson <hpj cl no>
Date:   Fri Nov 25 15:42:01 2016 +0100

    environment: Fix KeyError when CONFIG_SITE is unset.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=745223

 jhbuild/environment.py |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/jhbuild/environment.py b/jhbuild/environment.py
index 81d368c..029186e 100644
--- a/jhbuild/environment.py
+++ b/jhbuild/environment.py
@@ -116,7 +116,8 @@ def setup_env_defaults(system_libdirs):
                 valarr.remove(x)
         os.environ['LD_PRELOAD'] = ' '.join(valarr)
 
-    del os.environ['CONFIG_SITE']
+    if os.environ.has_key('CONFIG_SITE'):
+        del os.environ['CONFIG_SITE']
 
 def setup_env(prefix):
     '''set environment variables for using prefix'''