Finally fix parallel builds

* configure.ac (gpgme): Add assuan dependency.
* m4/gpg4win.m4 (GPG4WIN_SPKGEX): Do call GPG4WIN_SPKG here.

You can now do top level parallel builds so that for example
Qt is built parallel with boost.

The problem was that calling the GPG4WIN_SPKG template
in the SPKGEX macro overwrote the dependencies of the
32 bit package. This broke GPGME as the 32 bit GPGME
no longer depended on glib and qt.