prepare skia for shared library build on android

gyp/common.gypi

 # Copyright 2011 The Android Open Source Project
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # This file is automatically included by gyp_skia when building any target.
 
 {
   'includes': [
     'common_variables.gypi',
@@ skipping 21 matching lines @@
           'error': '<!(skia_angle=1 only supported with skia_os="win".)',
         }],
         [ 'skia_arch_width != 32 and skia_arch_width != 64', {
           'error': '<!(skia_arch_width can only be 32 or 64 bits not <(skia_arch_width) bits)',
         }],
         [ 'skia_os == "nacl" and OS != "linux"', {
           'error': '<!(Skia NaCl build only currently supported on Linux.)',
         }],
       ],
     },
+    # Validate the 'skia_os' setting against 'skia_shared_lib', because shared
+    # library build is only supported on Android.
+    'variables': {
+      'conditions': [
+        [ 'skia_os != "android" and skia_shared_lib', {
+          'error': '<!(Skia shared lib build only currently supported on Android.)',
+        }],
+      ],
+    },
     'includes': [
       'common_conditions.gypi',
     ],
     'conditions': [
       [ 'skia_scalar == "float"',
         {
           'defines': [
             'SK_SCALAR_IS_FLOAT',
             'SK_CAN_USE_FLOAT',
           ],
@@ skipping 53 matching lines @@
         ],
       },
     },
   }, # end 'target_defaults'
 }
 # Local Variables:
 # tab-width:2
 # indent-tabs-mode:nil
 # End:
 # vim: set expandtab tabstop=2 shiftwidth=2: