Try to fix python shlib probe for MinGW.

Per discussion with Andrew Dunstan, it seems there are three peculiarities
of the Python installation on MinGW (or at least, of the instance on
buildfarm member frogmouth).  First, the library name doesn't contain
"2.7" but just "27".  It looks like we can deal with that by consulting
get_config_vars('VERSION') to see whether a dot should be used or not.
Second, the library might be in c:/Windows/System32, analogously to it
possibly being in /usr/lib on Unix-oid platforms.  Third, it's apparently
not standard to use the prefix "lib" on the file name.  This patch will
accept files with or without "lib", but the first part of that may well
be dead code.