travis.yml: work around Go 1.9.4 issue (#16082)

* travis.yml: work around Go 1.9.4 issue

* travis: workaround the workaround

Author: fjl

.travis.yml

@@ -185,6 +185,8 @@ matrix:
         - xctool -version
         - xcrun simctl list
 
+        # Workaround for https://github.com/golang/go/issues/23749
+        - export CGO_CFLAGS_ALLOW='-fmodules|-fblocks|-fobjc-arc'
         - go run build/ci.go xcode -signer IOS_SIGNING_KEY -deploy trunk -upload gethstore/builds
 
     # This builder does the Azure archive purges to avoid accumulating junk