another kludge-- bring back the code in makefile.mingw to compile everything in extra, and remove the expr~ external files

add x_preset.c and x_magicglass.c back to makefile.mingw-- they were accidentally removed in the big Vanilla backport

port c6771297b6f03887034c5f121d3dbf701623ab87 from Pd-l2ork: the big Vanilla forward port won't compile without this commit

add another header (not sure if it's supposed to be there, but mingw complains about missing symbols)