Add mingw32 support.

Add mingw32 support.

        * pexecute.c (pexecute): New function for mingw32. Supports pipes.
        (pwait): New function for mingw32.
        * config.table (i[3456]86-*-mingw32*): Support for i386-mingw32.
        * config/mt-mingw32: New file.
        * xmalloc.c (first_break): Not used for mingw32.
        (xmalloc_set_program_name): Don't use sbrk on mingw32.
        (xmalloc): Likewise.
        (xrealloc): Likewise.

Co-Authored-By: J.J. VanderHeijden <J.J.vanderHeijden@student.utwente.nl>
From-SVN: r17395