Fall back to system GTest if available (mfontanini#473)

Using a submodule to download and build GTest is a great approach for
most circumstances, but some may prefer to use the system-provided GTest
if it is available.

This change adds a fallback to using the system's GTest if the submodule
is absent.

Author: cottsay

CMakeLists.txt

@@ -303,7 +303,13 @@ IF(LIBTINS_BUILD_TESTS)
         ENABLE_TESTING()
         ADD_SUBDIRECTORY(tests)
     ELSE()
-        MESSAGE(STATUS "googletest git submodule is absent. Run `git submodule init && git submodule update` to get it")
+        FIND_PACKAGE(GTest QUIET)
+        IF(${GTest_FOUND})
+            ENABLE_TESTING()
+            ADD_SUBDIRECTORY(tests)
+        ELSE()
+            MESSAGE(STATUS "googletest git submodule is absent. Run `git submodule init && git submodule update` to get it")
+        ENDIF()
     ENDIF()
 ENDIF()