Bug 1387388 - Disable -Wimplicit-fallthrough when building jsdtoa.cpp (exists now with gcc 7) r=nbp

MozReview-Commit-ID: 7YuX7SMePOi

--HG--
extra : rebase_source : d35a56f2b453bcd6a2e65738d96606d11cd3028b

js/src/moz.build

@@ -737,7 +737,7 @@ if CONFIG['GNU_CXX']:
     CXXFLAGS += ['-Wno-shadow', '-Werror=format', '-fno-strict-aliasing']
 
 # Suppress warnings in third-party code.
-if CONFIG['CLANG_CXX']:
+if CONFIG['CLANG_CXX'] or CONFIG['GNU_CXX']:
     SOURCES['jsdtoa.cpp'].flags += ['-Wno-implicit-fallthrough']
 
 # Generate GC statistics phase data.