Add autoconf macro to check for minimum libmicrohttpd version

* m4/mhd.m4: New.

m4/mhd.m4

@@ -0,0 +1,49 @@
+# mhd.m4
+
+#  This file is part of TALER
+#  Copyright (C) 2022 Taler Systems SA
+#
+#  TALER is free software; you can redistribute it and/or modify it under the
+#  terms of the GNU General Public License as published by the Free Software
+#  Foundation; either version 3, or (at your option) any later version.
+#
+#  TALER is distributed in the hope that it will be useful, but WITHOUT ANY
+#  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+#  A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License along with
+#  TALER; see the file COPYING.  If not, If not, see <http://www.gnu.org/license>
+
+# serial 1
+
+dnl MHD_VERSION_AT_LEAST([VERSION])
+dnl
+dnl Check that microhttpd.h can be used to build a program that prints out
+dnl the MHD_VERSION tuple in X.Y.Z format, and that X.Y.Z is greater or equal
+dnl to VERSION.  If not, display message and cause the configure script to
+dnl exit failurefully.
+dnl
+dnl This uses AX_COMPARE_VERSION to do the job.
+dnl It sets shell var mhd_cv_version, as well.
+dnl
+AC_DEFUN([MHD_VERSION_AT_LEAST],
+[AC_CACHE_CHECK([libmicrohttpd version],[mhd_cv_version],
+ [AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+  #include <stdio.h>
+  #include <microhttpd.h>
+]],[[
+  int v = MHD_VERSION;
+  printf ("%x.%x.%x\n",
+          (v >> 24) & 0xff,
+          (v >> 16) & 0xff,
+          (v >>  8) & 0xff);
+]])],
+  [mhd_cv_version=$(./conftest)],
+  [mhd_cv_version=0])])
+AX_COMPARE_VERSION([$mhd_cv_version],[ge],[$1],,
+  [AC_MSG_ERROR([[
+***
+*** You need libmicrohttpd >= $1 to build this program.
+*** ]])])])
+
+# mhd.m4 ends here