[CMake] Managing warning level

[jbd]

Hello,

Sorry to post this question, but i didn't find any answer to it.

I'm wondering how to manage the warning level of a compilation
process. Do i have to do some ugly IF(WIN32) foobar ? I'd like to do
it the abstract way, since i need to compile on windows with mingw, visual 
and on GNU/Linux using gcc.

I've seen that there is a variable that can be used for setting
warning level (CMAKE_CXX_WARNING_LEVEL), but it seems to work only with
Visual.

I'm sure i'm missing something =)

Thank you !