Add UPnP support.

Feedback is in logs as suggested by skid_au. The checkbox is still there, but
mostly for people who would like to opt out (unfortunately, I can not be sure
how this feature may behave for some routers - there's a hell of a lot of bad
UPnP implementations.)

The Visual Studio stuff is a little messy, so I apologize if anything is a bit
off. I tested most configurations and it worked.

I also tested CMake on Debian Wheezy, Ubuntu Saucy, and Mac OS X Mountain Lion.
All seemed to be OK.

CMakeLists.txt

@@ -9,6 +9,7 @@ option(USE_X11 "Enables X11 Support" ON)
 option(USE_WAYLAND "Enables Wayland Support" OFF)
 option(USE_GLES "Enables GLES2 And EGL, disables OGL" OFF)
 option(USE_GLES3 "Enables GLES3 and EGL" OFF)
+option(USE_UPNP "Enables UPnP port mapping support" ON)
 option(DISABLE_WX "Disable wxWidgets (use CLI interface)" OFF)
 
 option(FASTLOG "Enable all logs" OFF)
@@ -549,6 +550,13 @@ else()
        include_directories(Externals/SFML/include)
 endif()
 
+if(USE_UPNP)
+	message("Using static MiniUPnP client from Externals")
+	add_subdirectory(Externals/miniupnpc)
+	include_directories(Externals/miniupnpc/src)
+	add_definitions(-DUSE_UPNP)
+endif()
+
 if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND NOT ANDROID)
 	check_lib(SOIL SOIL SOIL/SOIL.h QUIET)
 endif()