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
With this we're not longer exporting the constructor headers, which means
we're not tying ourselves to a macro that might need special tweaking on
a compiler-by-compiler basis.
