Make build/install more user and packaging friendly

Make the build and install more user and packaging friendly
by introducing the following features in the Makefile:

  * Honor the $(CFLAGS) set when calling make.

  * Prefix all install locations with $(DESTDIR) for
    easy package building.

  * Use GNU Makefile conventions for defining installation
    directories. This means $(bindir) and $(man1dir)
    instead of the former $(INSTALL_DIR) and $(MAN_DIR).

  * Use install(1) for installing files and directories
    so that permissions can be set properly.

  * Stop "make clean" failing when it has nothing to do.

  * Add 'uninstall' make target.

  * Make 'install' target build executable if necessary.