gconstructor: Work around constructors being optimized away on MSVC
gconstructor: Work around constructors being optimized away on MSVC

Whole program optimization is enabled by default in visual studio
release builds, and this causes constructors (for e.g. resources) to be
optimized away as they are not referenced from elsewhere.

This works around this by some pragma magic.

https://bugzilla.gnome.org/show_bug.cgi?id=752837

mzabaluev/glib,endlessm/glib,MathieuDuponchelle/glib,Distrotech/glib,mzabaluev/glib,johne53/MB3Glib,mzabaluev/glib,johne53/MB3Glib,Distrotech/glib,mzabaluev/glib,MathieuDuponchelle/glib,ieei/glib,johne53/MB3Glib,Distrotech/glib,johne53/MB3Glib,ieei/glib,endlessm/glib,ieei/glib,ieei/glib,johne53/MB3Glib,Distrotech/glib,endlessm/glib,Distrotech/glib,endlessm/glib,MathieuDuponchelle/glib,endlessm/glib,MathieuDuponchelle/glib,johne53/MB3Glib,mzabaluev/glib,ieei/glib,MathieuDuponchelle/glib