Make build/install more packaging friendly

Make the build and install more packaging friendly:

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

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

  * Name install dir for executable $(BIN_DIR) which is
    more in line with the usual GNU $(bindir) than the
    previous unspecific name $(INSTALL_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.