This moves the external to gpgmepp, since that's what we _actually_
link against; plus tons of enablement for Windows build, mostly
related to linker probs integration.

There's still no good way to build a DLL with autotools, so we fall
back to gbuild manual make, see also tdf#91480

Change-Id: Ifd8217ef58536612d2389d48e343db133a13fb9c
Reviewed-on: https://gerrit.libreoffice.org/44970Reviewed-by: Siegmund Gorr <siegmund.gorr@cib.de>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>