Makefile: use vars for CC/CFLAGS, add -f to rm.

The Makefile had the C compiler hardcoded to gcc and CFLAGS hard
coded to -Wall. To use another compiler or different flags, one
would have had to edit the Makefile. This patch moves those values
into variables. This allows the user to override them from the
command line. For example, to compile with clang one could do
`make CC=clang`.

This patch also adds '-f' to the rm command so that there won't be
errors if the files don't exist.