build system: fix for MinGW

git-svn-id: https://www.dynare.org/svn/dynare/trunk@3230 ac1d8469-bf42-47a9-8791-bf33cf982152

m4/ax_mexext.m4

@@ -46,7 +46,7 @@ else
         ax_cv_mexext=`$MATLAB/bin/mexext.bat | sed 's/\r//'`
         ;;
       *mingw*)
-        ax_cv_mexext=`cd $MATLAB && cmd /c mexext.bat | sed 's/\r//'`
+        ax_cv_mexext=`cd $MATLAB/bin && cmd /c mexext.bat | sed 's/\r//'`
         ;;
       *)
         ax_cv_mexext=`$MATLAB/bin/mexext`