I'm assuming that you're using Make or something similar. I would create two files that contain different implementations of the main function, then in the makefile, define two separate targets that have identical dependencies on the rest of your files, except one uses your "unit test main" and the other your "normal main". Something like this:
normal: main_normal.c file1.c file2.c
unittest: main_unittest.c file1.c file2.c
As long as the "normal" target is closer to the top of the makefile, then typing "make" will choose it by default. You would have to type "make unittest" to build your test target.
