GN: spin off :no_exceptions as its own config.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4001

Change-Id: Ibaa10b9b6f566958b7e6622eedbc30a7010260b3
Reviewed-on: https://skia-review.googlesource.com/4001
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index cf092a2..bebe3c5 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -103,7 +103,6 @@
     ]
     cflags_cc += [
       "-std=c++11",
-      "-fno-exceptions",
       "-fno-threadsafe-statics",
       "-fvisibility-inlines-hidden",
     ]
@@ -210,6 +209,13 @@
   }
 }
 
+config("no_exceptions") {
+  # Exceptions are disabled by default on Windows.  (Use /EHsc to enable them.)
+  if (!is_win) {
+    cflags_cc = [ "-fno-exceptions" ]
+  }
+}
+
 config("warnings") {
   cflags = []
   cflags_cc = []