Add a convenient USEPCRE knob to the CMake support.
Add a convenient USEPCRE knob to the CMake support.

Change-Id: Iafb70f3bb037ea570400fa7766663048fd3a66db
Reviewed-on: https://code-review.googlesource.com/3570
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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