pw_build_info: Better flag compatibility
pw_build_info: Better flag compatibility

Conditionally applies a GCC-only flag when building with gcc.

Change-Id: I22d8decbd780db89b3b07c6880ff4f46a185897f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/66784
Pigweed-Auto-Submit: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed