Make the gflags library optional in the CMake build

When detected, add the necessary defines to actually use it.

Change-Id: I540c3e11e480be8aaab154ad91ee08cdc52319de
Reviewed-on: https://chromium-review.googlesource.com/988432
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: Frank Barchard <fbarchard@chromium.org>

CMakeLists.txt

@@ -72,7 +72,11 @@ if(TEST)
     target_link_libraries(libyuv_unittest glibc-compat)
   endif()
 
+  find_library(GFLAGS_LIBRARY gflags)
+  if(NOT GFLAGS_LIBRARY STREQUAL "GFLAGS_LIBRARY-NOTFOUND")
     target_link_libraries(libyuv_unittest gflags)
+    add_definitions(-DLIBYUV_USE_GFLAGS)
+  endif()
 endif()