[CMake] modify cmake build arguments

[hex]

I am still facing a problem: The build step is skipped only the link 
step is performed (I missed that in the beginning). It seems like the 
build step does not have any dependencies.


This is because different file extension are used. I cannot set 
CMAKE_C_SOURCE_FILE_EXTENSIONS because I believe `project` redefines the 
variable. If I set it afterwards it does not take effect. For now it 
works if I change the source file extension.