optionally use clang as

Flutter will need this to roll to Clang 12.

Change-Id: Ib341423e3ae3dcab933c17c27611e3d647dd3830
Reviewed-on: https://dart-review.googlesource.com/c/boringssl_gen/+/179001
Reviewed-by: Zach Anderson <zra@google.com>
Commit-Queue: Dan Field <dnfield@google.com>
1 file changed