NMake Makefiles: "Install" files in codegen/m4
NMake Makefiles: "Install" files in codegen/m4

This way, we could have the *.m4 files in place so that we can use them
to build gtkmm directly from a GIT checkout.

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