Fix detection of MATLAB MEX extension under Windows/MSYS2

This check recently broke under my MSYS2 installation. I’m unsure why, but I
guess it’s related to some Windows 10 update.

The solution is to stop calling it through “cmd”, and to call it directly as an
executable.

m4/ax_mexext.m4

@@ -1,7 +1,7 @@
 dnl ax_mexext.m4 --- check for MEX-file suffix.
 dnl
 dnl Copyright © 2000-2003 Ralph Schleicher
-dnl Copyright © 2009 Dynare Team
+dnl Copyright © 2009-2019 Dynare Team
 dnl
 dnl This program is free software; you can redistribute it and/or
 dnl modify it under the terms of the GNU General Public License as
@@ -40,12 +40,9 @@ else
     # The mexext script appeared in MATLAB 7.1
     AX_COMPARE_VERSION([$MATLAB_VERSION], [lt], [7.1], [AC_MSG_ERROR([I can't determine the MEX file extension. Please explicitly indicate it to the configure script with the MEXEXT variable.])])
     case $build_os in
-      *cygwin*)
+      *cygwin*|*mingw*)
         ax_cv_mexext=$("$MATLAB"/bin/mexext.bat | sed 's/\r//')
         ;;
-      *mingw*)
-        ax_cv_mexext=$(cd "$MATLAB"/bin && cmd /c mexext.bat | sed 's/\r//')
-        ;;
       *)
         ax_cv_mexext=$("$MATLAB"/bin/mexext)
         ;;