build: Use libmd auotmatically if available

Disable it for Debian packages for now, as this would pull the library
into the pseudo-essential set and requires discussion in debian-devel.

README

@@ -66,6 +66,7 @@ The minimum software required to configure and build dpkg from a tarball is:
 
 To enable optional functionality or programs, this software might be needed:
 
+  libmd (used by libdpkg, currently falling back to embedded code)
   zlib (used instead of gzip command-line tool)
   xz's liblzma (used instead of xz command-line tool)
   libbzip2 (used instead of bzip2 command-line tool)

debian/changelog

@@ -82,8 +82,11 @@ dpkg (1.18.8) UNRELEASED; urgency=medium
     - Preset Last-Update field in patch header template with current time in
       Dpkg::Source::Package::V2. Thanks to Daniel Shahaf <danielsh@apache.org>.
       Closes: #828146
+  * Packaging:
+    - Disable libmd usage in Debian and derivatives for now.
   * Build system:
     - Stop allowing to set deprecated bzip2 compressor as dpkg-deb default.
+    - Use libmd automatically if available.
   * Test suite:
     - Bump perlcritic ValuesAndExpressions::RequireNumberSeparators minimum
       to 99999.

debian/rules

@@ -59,6 +59,7 @@ build-tree/config.status: configure
 		--sbindir=/sbin \
 		--localstatedir=/var \
 		--with-devlibdir=\$${prefix}/lib/$(DEB_HOST_MULTIARCH) \
+		--without-libmd \
 		--with-zlib \
 		--with-liblzma \
 		--with-bz2

m4/dpkg-libs.m4

@@ -10,10 +10,6 @@ AC_DEFUN([DPKG_LIB_MD], [
   AC_ARG_WITH([libmd], [
     AS_HELP_STRING([--with-libmd],
                    [use libmd library for message digest functions])
-  ], [
-    with_libmd="$withval"
-  ], [
-    with_libmd="no"
   ])
   if test "x$with_libmd" != "xno"; then
     AC_CHECK_HEADERS([md5.h], [