Update the NMake Makefiles to invoke glib-compile-resources with
'start /min' so that when gdk-pixbuf-pixdata, json-glib-format
and xmllint are invoked by glib-compile-resources, cmd.exe windows
will not popup whenever these auxiliary tools are invoked, when one
builds GTK with the Visual Studio projects.
This will reduce the distraction that is caused during the time the
source files are generated, as a popup cmd.exe window will disrupt the
window focus as they appear, and will make the process a bit quicker.
