[AD] Getting Allegro for MSVC to build with Cygwin

["AE" <ae-a@xxxxxxxxxx>]

I have now made it possible to build Allegro for MSVC with Cygwin.

Attatched is a diff of allegro\makefile.vc from version 4.1.9 and a diff of
allegro\docs\build\msvc.txt from version 4.1.9

msvc.txt now explains how to build Allegro for MSVC using Cygwin.
makefile.vc now checks for the environment variable ALLEGRO_USE_CYGWIN and
if it's present, sets UNIX_TOOLS to be 1 and when compiling runner.exe, it
passes -mno-cygwin to GCC (question: Is there a reason why runner.exe is
compiled with GCC instead of MSVC?)

In mscv.txt, it says "Allegro should work ok with MSVC versions 4, 5 and 6".
Should we include version 7 too?

Do the makefiles and docs for building Allegro for other Windows or DOS
compilers such as BCC or Watcom need changing too to work with Cygwin? Are
there any other DOS/Windows based compilers supported by Allegro?

AE.

Attachment: makefile.vc.diff
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Attachment: msvc.txt.diff
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