add SDL2 sources

thirdparty/SDL2-src/acinclude/ac_check_define.m4

@@ -0,0 +1,14 @@
+AC_DEFUN([AC_CHECK_DEFINE],[dnl
+  AC_CACHE_CHECK(for $1 in $2, ac_cv_define_$1,
+    AC_EGREP_CPP([YES_IS_DEFINED], [
+#include <$2>
+#ifdef $1
+YES_IS_DEFINED
+#endif
+    ], ac_cv_define_$1=yes, ac_cv_define_$1=no)
+  )
+  if test "$ac_cv_define_$1" = "yes" ; then
+    AC_DEFINE([HAVE_$1],[],[Added by AC_CHECK_DEFINE])
+  fi
+])dnl
+AC_DEFINE([HAVE_$1],[],[Added by AC_CHECK_DEFINE])