make overriding CFLAGS easier

also make the CC invocations more consistent

Signed-off-by: Benedikt Morbach <benedikt.morbach@googlemail.com>
add install target

Signed-off-by: Benedikt Morbach <benedikt.morbach@googlemail.com>
Merge pull request #15 from moben/makefile_cleanup

Makefile cleanup: make installable and make CFLAGS easily overridable