Expose the USEPCRE knob as a CMake GUI option.

Change-Id: I1b2b4196273e3dac123aa772b9baf5ab1290cab0
Reviewed-on: https://code-review.googlesource.com/3572
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2c10a4d..be700f9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -6,6 +6,7 @@
 cmake_minimum_required(VERSION 2.8.7)
 
 project(RE2 CXX)
+option(USEPCRE "use PCRE in tests and benchmarks" OFF)
 
 set(EXTRA_TARGET_LINK_LIBRARIES)