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

[bil at ccrma.Stanford.EDU]

> Did you mean `so-file-name' actually?

Yes, thanks.

> libc.scm does not work because of lack of functions

Which functions?  It should be possible to comment them out in the msys 
case.

> but s7 does not know we switched to MING64

We need to make up our minds -- is msys2 actually different from mingw?  
If not,
let's use mingw64 as the compile time switch, not msys.