Only print Visual Studio messages when compiling with MSVC

When compiling with other compilers than MSVC, error messages about
C++11 support for versions below MSVC version 19.0.24210.0 were printed.
This happens, for example, when using clang-cl installed with the Visual
Studio Installer.

dlib/cmake_utils/set_compiler_specific_options.cmake

@@ -29,7 +29,7 @@ endif()
 set(USING_OLD_VISUAL_STUDIO_COMPILER 0)
 if(MSVC AND MSVC_VERSION VERSION_LESS 1900)
    message(FATAL_ERROR "C++11 is required to use dlib, but the version of Visual Studio you are using is too old and doesn't support C++11.  You need Visual Studio 2015 or newer. ")
-elseif(MSVC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0.24210.0 ) 
+elseif(MSVC AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0.24210.0 AND "MSVC" MATCHES ${CMAKE_CXX_COMPILER_ID})
    message(STATUS "NOTE: Visual Studio didn't have good enough C++11 support until Visual Studio 2015 update 3 (v19.0.24210.0)")
    message(STATUS "So we aren't enabling things that require full C++11 support (e.g. the deep learning tools).")
    message(STATUS "Also, be aware that Visual Studio's version naming is confusing, in particular, there are multiple versions of 'update 3'")