Add preliminary support for VR/OpenVR for HTC Vive and Oculus Rift,

(Windows-only first, later Linux and OSX)
Use premake to build, use --enable_openvr flag

build3/premake4.lua

@@ -29,6 +29,11 @@
 		description = "Try to link and use the system OpenGL headers version instead of dynamically loading OpenGL (dlopen is default)"
 	}
 
+	newoption
+	{
+		trigger = "enable_openvr",
+		description = "Enable experimental Virtual Reality examples, using OpenVR for HTC Vive and Oculus Rift"
+	}
 	newoption
 	{
 		trigger = "enable_system_x11",