make: allow to overwrite CFLAGS, CCACHE

Allow to define custom compile flags, e.g. ...

  make CFLAGS="-O2 -Wall"

... and also allow to overwrite ccache variable:

  make CCACHE= all

Also do some minor fixes when built with -O2 -Wall.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>