In order to make mtm easier to compile on a variety of operating
systems, let's use more standard makefile flags:

- Use CFLAGS for compiler flags.
- Use CPPFLAGS for preprocessor flags.
- Use LDFLAGS for linker flags.
- Deprecate LIBS in favor of LIBRARIES. It is better to avoid the use of
  LIBS for custom values as it is often used to store information for
  the linker.
- Deprecate LIBPATH. Use LDFLAGS to specify directories to search for
  libraries.

This way, for example, mtm can be compiled on FreeBSD with the following
command:

    env CFLAGS="-I/usr/local/include -I/usr/local/include/ncurses" \
            LDFLAGS=-L/usr/local/lib \
            make clean all

Also, I did not touch the HEADERS variable as I do not understand its
purpose.