[CMake] using llvm as a c compiler

[brad clawsie]

hi. 

i'm a cmake newbie. i am looking for some spoonfeeding regarding
changing my default c compiler to llvm-gcc, and how to set flags to
it. for example:

llvm-gcc -O3 -emit-llvm hello.c -c -o hello.bc

to compile a file hello.c

hopefully this message is not appearing twice, i sent it first prior
to joining the list properly

thanks