diff -pruN mozc-2.25.4190.102.orig/gyp/common.gypi mozc-2.25.4190.102/gyp/common.gypi
--- mozc-2.25.4190.102.orig/gyp/common.gypi	2020-11-19 20:22:49.310821694 +0900
+++ mozc-2.25.4190.102/gyp/common.gypi	2020-11-19 20:23:04.162662541 +0900
@@ -74,8 +74,6 @@
       '-Wwrite-strings',
       '<@(extra_warning_cflags)',
 
-      '-Wno-unknown-warning-option',
-      '-Wno-inconsistent-missing-override',
     ],
 
     # gcc_cflags will be shared with Mac and Linux
@@ -105,7 +103,6 @@
     ],
     # Libraries for GNU/Linux environment.
     'linux_ldflags': [
-      '-lc++',
       '-pthread',
       '-Wl,--build-id',
     ],
@@ -209,12 +206,13 @@
               '-Wno-covered-switch-default',
               '-Wno-unnamed-type-template-args',
               '-Wno-c++11-narrowing',
+              '-std=c++17',
+              '-stdlib=libc++',
             ],
           }],
-          ['compiler_target=="clang" or compiler_target=="gcc"', {
+          ['compiler_target=="gcc"', {
             'cflags_cc': [
               '-std=c++17',
-              '-stdlib=libc++',
             ],
           }],
         ],
@@ -229,12 +227,13 @@
               '-Wno-covered-switch-default',
               '-Wno-unnamed-type-template-args',
               '-Wno-c++11-narrowing',
+              '-std=c++17',
+              '-stdlib=libc++',
             ],
           }],
-          ['compiler_host=="clang" or compiler_host=="gcc"', {
+          ['compiler_host=="gcc"', {
             'cflags_cc': [
               '-std=c++17',
-              '-stdlib=libc++',
             ],
           }],
         ],