Fix ninja build on Android

R=mtklein@google.com

Review URL: https://codereview.chromium.org/25535002

git-svn-id: http://skia.googlecode.com/svn/trunk@11555 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gyp_skia b/gyp_skia
index 9c31978..b4f9e74 100755
--- a/gyp_skia
+++ b/gyp_skia
@@ -64,7 +64,7 @@
 
   if (sys.platform.startswith('darwin') and 
       (not os.getenv('GYP_GENERATORS') or 
-       'make' not in os.getenv('GYP_GENERATORS'))):
+       'xcode' in os.getenv('GYP_GENERATORS'))):
     print 'ERROR: variable SKIA_OUT is not valid on Mac (using xcodebuild)'
     sys.exit(-1);