Got a bare-bones version of SDL compiling for Windows RT. Dummy drivers are used in some places. Very little Windows-specific code (from the Win32 version of SDL) is used.

include/SDL_stdinc.h

@@ -72,6 +72,13 @@
 # include <ctype.h>
 #endif
 #ifdef HAVE_MATH_H
+# if defined(__WINRT__)
+/* Defining _USE_MATH_DEFINES is required to get M_PI to be defined on
+   Windows RT.  See http://msdn.microsoft.com/en-us/library/4hwaceh6.aspx
+   for more information.
+*/
+#  define _USE_MATH_DEFINES
+# endif
 # include <math.h>
 #endif
 #if defined(HAVE_ICONV) && defined(HAVE_ICONV_H)