allow building GDB with Python support on MinGW
    
    This makes several adjustements to the configure python-config.py
    scripts to deal with the differences between a Unix install of Python
    and a Windows install of Python (as downloaded from the Python website).
    
    Differences:
    
      - The Python executable is directly in the python prefix directory
        as opposed to inside the bin/ subdirectory.
    
      - The name of the python library is does not have a dot in the version
        number: On Unix, we have libpython2.7, while on Windows, it's
        libpython27.  So the regexp extracting the python version from
        the Python lib filename had to be adjusted slightly.
    
        Also, the tests checking the name of the libpython had to be
        adjusted to allow for that.
    
      - There are no link options following the -lpython<version> switch
        on Windows, but the regexp extracting the python version was
        using it as a delimiter.  It had to be removed.
    
      - python-config.py does not work on Windows, mostly because
        some sysconfig variables are missing.  They are not necessary
        so the script was adapted to skip them if not defined.
    
      - The paths returned by python-config.py follow the Windows filename
        convention in terms of the directory separator, and this is causing
        trouble when the build environment is cygwin (while the compiler
        and Python are MinGW).  We could have fixed that in the configure
        script, but it felt simpler to do so in python-config.py
    
    gdb/ChangeLog:
    
            * configure.ac: Add handling of Python distribution on Windows.
            * python-config.py: If the LIBS, SYSLIBS, LIBPL and/or LINKFORSHARED
            sysconfig variables are not defined, then do not use them.
            On Windows, if LIBPL is not defined, then use prefix + '/libs'
            instead.  On Windows, return all paths using forward-slashes
            rather than backslashes.