Download GTest if not found in system (#237)

Co-authored-by: Stanley Tsang <stanley.tsang@amd.com>

cmake/DownloadProject.CMakeLists.cmake.in

@@ -0,0 +1,14 @@
+# Distributed under the OSI-approved MIT License.  See accompanying
+# file LICENSE or https://github.com/Crascit/DownloadProject for details.
+
+cmake_minimum_required(VERSION 2.8.2)
+
+project(${DL_ARGS_PROJ}-download NONE)
+
+include(ExternalProject)
+ExternalProject_Add(${DL_ARGS_PROJ}-download
+                    ${DL_ARGS_UNPARSED_ARGUMENTS}
+                    SOURCE_DIR          "${DL_ARGS_SOURCE_DIR}"
+                    BUILD_IN_SOURCE     TRUE
+                    TEST_COMMAND        ""
+)