GN: Make wtf_unittests build on mac/clang

Source/BUILD.gn

Index: Source/BUILD.gn
diff --git a/Source/BUILD.gn b/Source/BUILD.gn
index 8911897298f3823ec21beda579ed3087688f05e8..002dabbcd93639a0d150dac6265327751db860ee 100644
--- a/Source/BUILD.gn
+++ b/Source/BUILD.gn
@@ -30,6 +30,8 @@ config("config") {
   cflags = []
   defines = []
 
+  configs = [ ":non_test_config" ]
+
   if (is_win) {
     cflags += [
       "/wd4305",  # Truncation from 'type1' to 'type2'.
@@ -60,7 +62,6 @@ config("config") {
   }
 
   if (is_clang) {
-    cflags += [ "-Wglobal-constructors" ]
     if (blink_gc_plugin && clang_use_chrome_plugins) {
       # TODO(GYP)
       #'cflags': ['<!@(../../../tools/clang/scripts/blink_gc_plugin_flags.sh enable-oilpan=<(enable_oilpan) dump-graph=<(blink_gc_plugin_dump_graph))'],
@@ -68,6 +69,16 @@ config("config") {
   }
 }
 
+# The follow configs apply to all targets except for unit tests, which rely on
+# static initializers.
+config("non_test_config") {
+  cflags = []
+
+  if (is_clang) {
+    cflags += [ "-Wglobal-constructors" ]
+  }
+}
+
 # stubs ------------------------------------------------------------------------
 
 # These set up include paths for targets that haven't been converted yet. They