https://github.com/google/mozc/issues/490

--- /src/gyp/defines.gypi
+++ /src/gyp/defines.gypi
@@ -67,6 +67,10 @@
     # use_system_abseil_cpp represents if system version or bundled version
     # of abseil-cpp library is used.
     'use_system_abseil_cpp%': '0',
+
+    # use_system_gtest represents if system version or bundled version
+    # of gtest library is used.
+    'use_system_gtest%': '0',
   },
   'target_defaults': {
     'defines': [
--- /src/testing/testing.gyp
+++ /src/testing/testing.gyp
@@ -59,54 +59,76 @@
   'targets': [
     {
       'target_name': 'testing',
-      'type': 'static_library',
-      'variables': {
-        'gtest_defines': [
-          'GTEST_LANG_CXX11=1',
-          'GTEST_HAS_TR1_TUPLE=0',  # disable tr1 tuple in favor of C++11 tuple.
-        ],
-        'gtest_dir': '<(third_party_dir)/gtest/googletest',
-        'gmock_dir': '<(third_party_dir)/gtest/googlemock',
-      },
-      'sources': [
-        '<(gmock_dir)/src/gmock-cardinalities.cc',
-        '<(gmock_dir)/src/gmock-internal-utils.cc',
-        '<(gmock_dir)/src/gmock-matchers.cc',
-        '<(gmock_dir)/src/gmock-spec-builders.cc',
-        '<(gmock_dir)/src/gmock.cc',
-        '<(gtest_dir)/src/gtest-death-test.cc',
-        '<(gtest_dir)/src/gtest-filepath.cc',
-        '<(gtest_dir)/src/gtest-matchers.cc',
-        '<(gtest_dir)/src/gtest-port.cc',
-        '<(gtest_dir)/src/gtest-printers.cc',
-        '<(gtest_dir)/src/gtest-test-part.cc',
-        '<(gtest_dir)/src/gtest-typed-test.cc',
-        '<(gtest_dir)/src/gtest.cc',
-      ],
-      'include_dirs': [
-        '<(gmock_dir)',
-        '<(gmock_dir)/include',
-        '<(gtest_dir)',
-        '<(gtest_dir)/include',
-      ],
-      'defines': [
-        '<@(gtest_defines)',
-      ],
-      'all_dependent_settings': {
-        'defines': [
-          '<@(gtest_defines)',
-        ],
-        'include_dirs': [
-          '<(gmock_dir)/include',
-          '<(gtest_dir)/include',
-        ],
-      },
       'conditions': [
-        ['(_toolset=="target" and compiler_target=="clang") or '
-         '(_toolset=="host" and compiler_host=="clang")', {
-          'cflags': [
-            '-Wno-missing-field-initializers',
-            '-Wno-unused-private-field',
+        ['use_system_gtest==1', {
+          'type': 'none',
+          'variables': {
+            'gtest_defines': [
+              'GTEST_LANG_CXX11=1',
+              'GTEST_HAS_TR1_TUPLE=0',  # disable tr1 tuple in favor of C++11 tuple.
+            ],
+          },
+          'all_dependent_settings': {
+            'defines': [
+              '<@(gtest_defines)',
+            ],
+            'link_settings': {
+              'libraries': [
+                '-lgmock -lgtest',
+              ],
+            },
+          },
+        }, {
+          'type': 'static_library',
+          'variables': {
+            'gtest_defines': [
+              'GTEST_LANG_CXX11=1',
+              'GTEST_HAS_TR1_TUPLE=0',  # disable tr1 tuple in favor of C++11 tuple.
+            ],
+            'gtest_dir': '<(third_party_dir)/gtest/googletest',
+            'gmock_dir': '<(third_party_dir)/gtest/googlemock',
+          },
+          'sources': [
+            '<(gmock_dir)/src/gmock-cardinalities.cc',
+            '<(gmock_dir)/src/gmock-internal-utils.cc',
+            '<(gmock_dir)/src/gmock-matchers.cc',
+            '<(gmock_dir)/src/gmock-spec-builders.cc',
+            '<(gmock_dir)/src/gmock.cc',
+            '<(gtest_dir)/src/gtest-death-test.cc',
+            '<(gtest_dir)/src/gtest-filepath.cc',
+            '<(gtest_dir)/src/gtest-matchers.cc',
+            '<(gtest_dir)/src/gtest-port.cc',
+            '<(gtest_dir)/src/gtest-printers.cc',
+            '<(gtest_dir)/src/gtest-test-part.cc',
+            '<(gtest_dir)/src/gtest-typed-test.cc',
+            '<(gtest_dir)/src/gtest.cc',
+          ],
+          'include_dirs': [
+            '<(gmock_dir)',
+            '<(gmock_dir)/include',
+            '<(gtest_dir)',
+            '<(gtest_dir)/include',
+          ],
+          'defines': [
+            '<@(gtest_defines)',
+          ],
+          'all_dependent_settings': {
+            'defines': [
+              '<@(gtest_defines)',
+            ],
+            'include_dirs': [
+              '<(gmock_dir)/include',
+              '<(gtest_dir)/include',
+            ],
+          },
+          'conditions': [
+            ['(_toolset=="target" and compiler_target=="clang") or '
+             '(_toolset=="host" and compiler_host=="clang")', {
+              'cflags': [
+                '-Wno-missing-field-initializers',
+                '-Wno-unused-private-field',
+              ],
+            }],
           ],
         }],
       ],