Some systems provide prebuilt copies of gmock/gtest (such as Chromium
OS).  Add a configure flag so they can take advantage of that.  This
allows for a smaller checkout as they don't need to include the full
testing/ tree.

BUG=chromium:579384
TEST=`make check` passes w/--enable-system-test-libs
TEST=`make check` passes w/--disable-system-test-libs
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1638653002 .