gyp: add osmesa target to factor out common code

gyp/gputest.gyp

Index: gyp/gputest.gyp
diff --git a/gyp/gputest.gyp b/gyp/gputest.gyp
index 2e905189cde735b0e1e60d2fa61d647ffa48d9d3..d1067e5f99719ed9b7a96ae54f22f60aa4f6ef66 100644
--- a/gyp/gputest.gyp
+++ b/gyp/gputest.gyp
@@ -32,23 +32,7 @@
         '<!@(python find.py ../tools/gpu "*")'
       ],
       'conditions': [
-        [ 'skia_mesa and skia_os == "linux"', {
-          'link_settings': {
-            'libraries': [
-              '-lOSMesa',
-            ],
-          },
-        }],
-        [ 'skia_mesa and skia_os == "mac"', {
-          'link_settings': {
-            'libraries': [
-              '/opt/X11/lib/libOSMesa.dylib',
-            ],
-          },
-          'include_dirs': [
-             '/opt/X11/include/',
-          ],
-        }],
+        [ 'skia_mesa', { 'dependencies': [ 'osmesa' ] } ],
         [ 'skia_angle', {
           'dependencies': [
             'angle.gyp:*',
@@ -121,5 +105,18 @@
         }],
       ],
     },
+    {
+      'target_name': 'osmesa',
+      'type': 'none',
+      'direct_dependent_settings': {
+        'link_settings': { 'libraries': [ '-lOSMesa', ], },
+        'conditions': [
+          [ 'skia_os == "mac"', {
+            'link_settings': { 'library_dirs' : [ '/opt/X11/lib' ], },
+            'include_dirs': [ '/opt/X11/include', ],
+          }],
+        ],
+      },
+    },
   ],
 }