Use latest warnings.m4 from gnulib
Use latest warnings.m4 from gnulib

The one we were using does not work properly with clang, causing
the build process to try to use -f/-W options that are not
supported by clang.

elmarco/spice-gtk,guodong/spice-gtk,flexVDI/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk