Nothing defines GTEST_USES_PCRE anymore. It was only meant for
internal use, so nothing public should be relying on it:
https://github.com/google/googletest/issues/2735#issuecomment-644849438.
Found when compiling with "-Wundef".
Fixes#2735.
PiperOrigin-RevId: 507823660
Change-Id: Ie19e576ff01dc3b16381338578ece92adccfc09b