Use makefile variables in a more standard manner

[0mp]

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.