build: Allow CC to be different than clang

This is useful to build with another compiler, say gcc.
Also, on DragonFly BSD, clang is not (yet) in base and without this
change, it prevents bsdfan from building (unless clang is installed of
course).
build: use c99

This allows building with gcc.
add gitignore file
Merge pull request #1 from Rolinh/master

Improve Makefile