build: Fix inversion logic in libmd configure check when requested explicitly

debian/changelog

@@ -3,6 +3,8 @@ dpkg (1.18.14) UNRELEASED; urgency=medium
   [ Guillem Jover ]
   * Test suite:
     - Do not fail tests on missing fakeroot, just skip them.
+  * Build system:
+    - Fix inversion logic in libmd configure check when requested explicitly.
 
   [ Updated man pages translations ]
   * German (Helge Kreutzmann).

m4/dpkg-libs.m4

@@ -19,7 +19,7 @@ AC_DEFUN([DPKG_LIB_MD], [
         have_libmd="yes"
       ])
     ])
-    AS_IF([test "x$have_libmd" = "xyes" && test "x$with_libmd" = "xyes"], [
+    AS_IF([test "x$with_libmd" = "xyes" && test "x$have_libmd" = "xno"], [
       AC_MSG_FAILURE([md5 digest not found in libmd])
     ])
   ])