Verifies two important properties of #includes in the RTS:

 * That system headers don't appear inside of a `<BeginPrivate.h>` block
   as this can hide system library symbols, resulting in very
   hard-to-diagnose linker errors

 * That no headers precede `Rts.h`, ensuring that __USE_MINGW_ANSI_STDIO
   is set correctly before system headers are included.