g15helper: statically link against runtime library on Windows.
g15helper: statically link against runtime library on Windows.

The mumble-g15-helper.exe binary is always 32-bit on Windows.
However, we don't ship 32-bit VCRedist for 64-bit Mumble builds.

This change forces us to always statically link mumble-g15-helper.exe
to ensure that it works on 64-bit systems without 32-bit VCRedist
manually installed.

Fixes mumble-voip/mumble#1692

unascribed/mumble,SuperNascher/mumble,Lartza/mumble,unascribed/mumble,SuperNascher/mumble,unascribed/mumble,LuAPi/mumble,Lartza/mumble,unascribed/mumble,unascribed/mumble,austinliou/mumble,LuAPi/mumble,unascribed/mumble,LuAPi/mumble,Lartza/mumble,Lartza/mumble,austinliou/mumble,LuAPi/mumble,Lartza/mumble,LuAPi/mumble,austinliou/mumble,SuperNascher/mumble,austinliou/mumble,SuperNascher/mumble,LuAPi/mumble,austinliou/mumble,Lartza/mumble,austinliou/mumble,LuAPi/mumble,SuperNascher/mumble,unascribed/mumble,SuperNascher/mumble,austinliou/mumble,austinliou/mumble,SuperNascher/mumble,SuperNascher/mumble,Lartza/mumble,Lartza/mumble,LuAPi/mumble,LuAPi/mumble,SuperNascher/mumble,unascribed/mumble