Explicitly don't support the make gyp generator.

build/gyp_chromium

Index: build/gyp_chromium
diff --git a/build/gyp_chromium b/build/gyp_chromium
index 97179c1b150b508da82d7c7e2a949b73ace15123..06e7cc74a1cc979258d31c4261cd635915a628ba 100755
--- a/build/gyp_chromium
+++ b/build/gyp_chromium
@@ -454,6 +454,12 @@ if __name__ == '__main__':
   if sys.platform not in ('darwin',):
     args.append('--no-circular-check')
 
+  # We explicitly don't support the make gyp generator (crbug.com/348686). Be
+  # nice and fail here, rather than choking in gyp.
+  if 'make' in os.environ.get('GYP_GENERATORS', ''):
+    print 'Error: make gyp generator not supported (check GYP_GENERATORS).'
+    sys.exit(1)
+
   # Default to ninja on linux and windows, but only if no generator has
   # explicitly been set.
   # Also default to ninja on mac, but only when not building chrome/ios.