Set up the use of {Windows,Apple}/{include,lib} regardless of feature toggles in Makefile.common using $(abspath ) in reference to the directory Makefile.common is in.
Add the three DirectX headers that are actually used to the repo. (from: http://alleg.sourceforge.net/files/dx9mgw.zip)
Since current MinGW versions include DirectX libs (for dynamic linking), remove "-L$(DXROOT)/lib".
The DirectX headers are no longer a separate dependency for building.
Add $(SDLROOT_OVERRIDE). $(SDLROOT) only functions for Windows and Mac OS X.
Factor handling of $(DXROOT_OVERRIDE) and $(SDLROOT_OVERRIDE) into Makefile.shared using $(abspath ).
git-svn-id: https://svn.eduke32.com/eduke32@3275 1a8010ca-5511-0410-912e-c29ae57300e0
