CMakeLists.txt: build gtest from a stable branch instead of master

cmake/Gtest.cmake.in

@@ -12,7 +12,9 @@ project(googletest-download NONE)
 include(ExternalProject)
 ExternalProject_Add(googletest
   GIT_REPOSITORY    https://github.com/google/googletest.git
-  GIT_TAG           master
+  #GIT_TAG           master
+  # build from a stable branch instead of master (which gets broken pretty often)
+  GIT_TAG           v1.10.x
   SOURCE_DIR        "${CMAKE_BINARY_DIR}/googletest-src"
   BINARY_DIR        "${CMAKE_BINARY_DIR}/googletest-build"
   CONFIGURE_COMMAND ""