Changes for MinGW/WIN32:

        o allow configure to see include/port/win32 include files
        o add matching Win32 accept() prototype
        o allow pg_id to compile with native Win32 API
        o fix invalide mbvalidate() function calls (existing bug)
        o allow /scripts to compile with native Win32 API
        o add win32.c to Win32 compiles (already in *.mak files)