Added three function to top-level Makefile:
cc-option (replacing check_gcc)
cc-option-yn (as cc-option but return y or n)
cc-version (numeric version of $(CC))
Documentation added to Documentation/kbuild/makefiles.txt

The check_gcc version is still present, but now spit out a arning if used.
The new naming where chosen to get consistent naming of all three functions
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>