Add .PHONY, use `install`, and obey DESTDIR
Use _DEFAULT_SOURCE instead of _BSD_SOURCE

Due to the ``Makefile``'s ``-Werror`` invocation, I get the following error when compiling:

    #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE" [-Werror=cpp]

This commit changes ``-D_BSD_SOURCE`` to ``-D_DEFAULT_SOURCE`` which solves the issue quite nicely.
Remove odd ``/`` ending PREFIX; create directories with ``-d`` first to be portable

Hopefully, this will enable the makefile to be compatible for both BSD and Linux ``install``s.
Remove seemingly useless recursive flag from the ``clean`` target
Merge branch 'HalosGhost-patch-1'