fix autotools build on windows

Like already done for cmake,
we need to add disc_win32_new.lo to the objects to build
with the autotools on Windows.

Tested with
./autogen.sh; ./configure; make; make install
on Cygwin and MinGW (both on a virtual WinXP)

cygwin output: cygdiscid-0.dll
mingw output: libdiscid-0.dll