Make `gn gen` work on Windows again.

Clang detection is specific to GCC-like toolchains.

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3171

Change-Id: I7144bc8e5cd3e774625b51a6dda981284ed1fdc1
Reviewed-on: https://skia-review.googlesource.com/3171
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index a193942..78be521 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -24,12 +24,14 @@
   cc_wrapper = ""
 }
 
-is_clang = exec_script("is_clang.py",
-                       [
-                         cc,
-                         cxx,
-                       ],
-                       "value")
+if (!is_win) {
+  is_clang = exec_script("is_clang.py",
+                         [
+                           cc,
+                           cxx,
+                         ],
+                         "value")
+}
 
 config("default") {
   asmflags = []