Use ExternalPoject_Add() to split by toolchain

cmake/DownloadGTest.cmake

@@ -0,0 +1,25 @@
+set(GTEST_ROOT /usr CACHE STRING "Base path to Google Test headers and source.")
+
+find_path(GTEST_INCDIR gtest/gtest.h HINTS "${GTEST_ROOT}/include" DOC "Path to Google Test header files folder ('gtest/gtest.h').")
+find_path(GTEST_SRCDIR src/gtest-all.cc HINTS "${GTEST_ROOT}" "${GTEST_ROOT}/src/gtest" DOC "Path of Google Test 'src' folder.")
+
+if(NOT GTEST_INCDIR OR NOT GTEST_SRCDIR)
+  message(STATUS "Googletest will be downloaded")
+
+  set(GTEST_VERSION 1.8.1)
+
+  set(GTEST_SRCDIR "${RADIO_SRC_DIR}/tests/googletest-release-${GTEST_VERSION}/googletest")
+  set(GTEST_INCDIR "${GTEST_SRCDIR}/include")
+
+  set(GTEST_URL     "https://github.com/google/googletest/archive/release-${GTEST_VERSION}.tar.gz")
+  set(GTEST_ARCHIVE "${RADIO_SRC_DIR}/tests/gtest-${GTEST_VERSION}.tar.gz")
+
+  if (NOT EXISTS "${GTEST_SRCDIR}")
+    file(DOWNLOAD "${GTEST_URL}" ${GTEST_ARCHIVE} SHOW_PROGRESS)
+    execute_process(
+      COMMAND ${CMAKE_COMMAND} -E tar -xf ${GTEST_ARCHIVE}
+      WORKING_DIRECTORY ${RADIO_SRC_DIR}/tests)
+    execute_process(
+      COMMAND ${CMAKE_COMMAND} -E remove -f ${GTEST_ARCHIVE})
+  endif()
+endif()