Patch against bison-1.875d: making it compile on Windows (mingw)

[Vassili Bourdo]

Hi there!

Here is the solution to dreaded fork problem on MS Windows (see attachment).
I replaced fork-exec sequence in create_subpipe() with CreateProcess() call
As result, I managed to compile and run bison under Mingw GCC.

Also, for convenience, I changed bison package location from 
{prefix}/share/bison to {bison.exe directory}/bison

Another problem I didnt solve is that under Mingw ./configure resolves m4
binary to '/bin/m4', but it should resolve it to 'm4.exe' or 'm4'.
Instead of fiddling with autoconf, I fix these matters in generated 'config.h' 
manually.

Issues still open:
- I rely on _WIN32 macro. But I dont know whether CYGWIN defines this macro.
   If it does, it would make sence to disable my patch for cygwin and eable
   it for Mingw only ...

bison.patch
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