NMake Makefiles: Add option to use Meson-built libsigc++
NMake Makefiles: Add option to use Meson-built libsigc++

This makes it easier for people to build and link against Meson-built
libsigc++-3.x, by providing an NMake command line option.

GNOME/atkmm,GNOME/atkmm,GNOME/atkmm,GNOME/atkmm