gtk3-demo, gtk3-demo-application and gtk3-icon-factory are built using
/opt:noref as they use GResources without --manual-register, meaning that
flag is used to avoid the linker from optimizing out the GResource items
for release builds.
Visual Studio 2015 changed its default link-time code generation setting
to /LTCG:incremental, which would cause problems as it will cause the
linker to optimize out the GResource items from the compiled code, so fix
this by enforcing /LTCG for these three items.
fb8c947091