OS2/cygwin/mingw support

[Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>]

Dear developers,

The code to handle PE platforms (os2,cygwin,mingw) has lot of duplication,
and since I only ever test mingw (using wine...) I cannot really fix them
without taking the risk of breaking cygwin/os2.
For example config/get_dlld has:

    cygwin) DLLDFLAGS="-Wl,--out-implib=\$(LIBPARI_SO)\$(_A),--export-all-symbols";;
    mingw) DLLDFLAGS="-Wl,--out-implib=\$(LIBPARI_SO)\$(_A)";;

I have no idea why we use --export-all-symbols on cygwin and not on mingw.

However the os2 code is worse in the sense it is almost a complete copy of
the cygwin code and is probably totally untested, so maybe we should get rid of
it.

Cheers,
Bill.