Use makefile variables in a more standard manner

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.

deadpixi / mtm