Most libcs seem to be reasonable. Bionic (Android) always makes
pthread_rwlock_t. Reportedly, NetBSD does too. Default to assuming libcs
are reasonable and treat glibc as the exception.
Update-Note: If there are non-glibc libcs with similarly problematic
headers, this may break the build. Let us know if it does.
Fixed: 482
Change-Id: I63052cad7693d9e28d98775205fe7688e262d88c
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/51685
Reviewed-by: Adam Langley <agl@google.com>