Visual Studio Builds: Provide Implementation for round()

Visual Studio 2012 and earlier do not provide round() in their CRTs, so
use the round() implementation in GTK+-3.x and add it in a compatibility
math.h header that is strictly for Visual Studio only, and let the Visual
Studio builds find this math.h first (which would then include the stock
math.h).

https://bugzilla.gnome.org/show_bug.cgi?id=739778