MI's slope compiling fixes

src/p_map.c

src/p_maputl.c

src/p_mobj.c

src/p_slopes.c

src/p_slopes.h

src/p_spec.c

src/p_user.c

src/r_bsp.c

src/r_draw.h

src/r_draw8.c

src/r_plane.c

src/r_segs.c

src/r_things.c