Allow using external absl and re2.

CMakeLists.txt

@@ -20,6 +20,15 @@ option(BUILD_GMOCK "Builds the googlemock subproject" ON)
 option(INSTALL_GTEST "Enable installation of googletest. (Projects embedding googletest may want to turn this OFF.)" ON)
 option(GTEST_HAS_ABSL "Use Abseil and RE2. Requires Abseil and RE2 to be separately added to the build." OFF)
 
+if(GTEST_HAS_ABSL)
+  if(NOT TARGET absl::base)
+    find_package(absl REQUIRED)
+  endif()
+  if(NOT TARGET re2::re2)
+    find_package(re2 REQUIRED)
+  endif()
+endif()
+
 if(BUILD_GMOCK)
   add_subdirectory( googlemock )
 else()