- introduce X11FLAVOUR to choose src/x11 vs src/external/mit/xorg
for the X11 to build if MKX11=yes is set. it takes the values
of either Xorg or XFree86.
- default to Xorg on alpha, i386, macppc, shark, sparc and sparc64
- remove MKXORG_WITH_XSRC_XSERVER, unused and never useful