[CM] a short guide on compile on msys2/windows with cload.scm

[bil at ccrma.Stanford.EDU]

Thanks very much!  I don't have a machine running Windows
or Msys2, but it appears that I can use the __MSYS__ compile
time flag to recognize it, then use that to add 'msys2
to s7's *features* list, and use that to fix cload.scm.
I'll try to do this later today.