[CMake] make CFLAGS= support

[Vladimir Prus]

Hello,
I have cmake version '2.6-patch 2', and after I generate makefiles,
and build with

	make CFLAGS=-O2

I don't see -O2 being used in the compiler command line. Is this
the expected behaviour?

I realize that CFLAGS is a automake convention, so I am not saying cmake 
should support it, but I wanted to check if maybe I have old CMake version,
or something.

Thanks,
Volodya