Add options to build CMake without any language dialects

If CMake_NO_<LANG>_STANDARD is set, do not set CMAKE_<LANG>_STANDARD.
This will allow users to build with their own -std= flags without
CMake adding any itself.

CMakeLists.txt

@@ -37,10 +37,10 @@ if("${CMake_SOURCE_DIR}" STREQUAL "${CMAKE_SOURCE_DIR}")
 endif()
 
 # Use most-recent available language dialects with GNU and Clang
-if(NOT DEFINED CMAKE_C_STANDARD)
+if(NOT DEFINED CMAKE_C_STANDARD AND NOT CMake_NO_C_STANDARD)
   set(CMAKE_C_STANDARD 11)
 endif()
-if(NOT DEFINED CMAKE_CXX_STANDARD)
+if(NOT DEFINED CMAKE_CXX_STANDARD AND NOT CMake_NO_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 14)
 endif()