Revert "Don't force CXX_STANDARD for Google Test."

This reverts commit 07a7331.

Author: gchatelet

CMakeLists.txt

@@ -211,6 +211,10 @@ if(BUILD_TESTING)
   # found.
   enable_language(CXX)
 
+  set(CMAKE_CXX_STANDARD 14)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_CXX_EXTENSIONS OFF) # prefer use of -std14 instead of -gnustd14
+
   if(NOT TARGET gtest OR NOT TARGET gmock_main)
     # Download and unpack googletest at configure time.
     configure_file(