games/glbsp: update the port to version 2.24 + miscellaneous fixes

• Catch up with new improved directory layout: the main code moved into the ``src'' subdirectory; existing makefiles had been fixed and consolidated so each type of target (lib/cmdline/gui) is now handled in the same makefile
• Drop PLUGIN option and simply install the header file and static library for consumer ports' convenience
• Install more complete set of documentation files while I'm at it