Meet AX_CHECK_LIB_USING_HEADER. Also threw X11 back in because why the hell not. configure actually completes successfully now.

autoconf/m4/ax_check_lib_using_header.m4

@@ -0,0 +1,24 @@
+dnl AX_CHECK_LIB_USING_HEADER(lib, function, header, [action-if-found], [action-if-not-found])
+dnl As AC_CHECK_LIB except "function" must be a fully valid function call and "header" the header file in which that function is defined. The benefit is that this can find functions whose symbols have been decorated, e.g. a lot of MinGW stuff.
+
+AC_DEFUN([AX_CHECK_LIB_USING_HEADER],
+[
+dnl	# On Windows OpenGL functions' library symbols don't match their function names. So we have to do it the hard way.
+	
+	AC_MSG_CHECKING(for $2 in $1 using $3)
+	
+	AC_LANG_PUSH(C)
+	
+	LIBS_SAVE=$LIBS
+	LIBS="$LIBS -l$1"
+	AC_LINK_IFELSE(
+		[AC_LANG_PROGRAM([#include <$3>], [$2])],
+		[AC_MSG_RESULT(yes)
+		$4],
+		[AC_MSG_RESULT(no)
+		$5]
+	)
+	LIBS=$LIBS_SAVE
+	
+	AC_LANG_POP(C)
+])