speedo,w32: Add extra flags for gpgrt and fix SO name of libassuan.

Due to the recently introduced use of STARTUPINFOEXW in gpgrt we now
need at least Windows Vista. Version 8 of Mingw defaults to XP SP2
which requires us to explicit override that default.

The SO number of libassuan needs an update too.

(cherry picked from commit d56b63a66181b4f029e91dcea095e9eb149895b7)