build: Handle libmd implementations built into system libc

As found on some BSD systems.

debian/changelog

@@ -33,6 +33,8 @@ dpkg (1.18.23) UNRELEASED; urgency=medium
   * Build system:
     - Check <sys/proc.h> by also including <sys/param.h>, on several BSD
       systems the header is not self-contained.
+    - Handle libmd implementations built into system libc, as found on some
+      BSD systems.
 
  -- Guillem Jover <guillem@debian.org>  Wed, 01 Feb 2017 06:28:28 +0100
 

m4/dpkg-libs.m4

@@ -14,16 +14,21 @@ AC_DEFUN([DPKG_LIB_MD], [
   have_libmd="no"
   AS_IF([test "x$with_libmd" != "xno"], [
     AC_CHECK_HEADERS([md5.h], [
-      AC_CHECK_LIB([md], [MD5Init], [
-        MD_LIBS="-lmd"
+      dpkg_save_libmd_LIBS=$LIBS
+      AC_SEARCH_LIBS([MD5Init], [md], [have_libmd="yes"])
+      LIBS=$dpkg_save_libmd_LIBS
+      AS_IF([test "x$ac_cv_search_MD5Init" = "xnone required"], [
+        have_libmd="bultin"
+      ], [test "x$ac_cv_search_MD5Init" != "xno"], [
         have_libmd="yes"
+        MD_LIBS="$ac_cv_search_MD5Init"
       ])
     ])
     AS_IF([test "x$with_libmd" = "xyes" && test "x$have_libmd" = "xno"], [
-      AC_MSG_FAILURE([md5 digest not found in libmd])
+      AC_MSG_FAILURE([md5 digest functions not found])
     ])
   ])
-  AM_CONDITIONAL([HAVE_LIBMD_MD5], [test "x$have_libmd" = "xyes"])
+  AM_CONDITIONAL([HAVE_LIBMD_MD5], [test "x$have_libmd" != "xno"])
 ])# DPKG_LIB_MD
 
 # DPKG_WITH_COMPRESS_LIB(NAME, HEADER, FUNC)