build: Do not fail on missing compression libraries or headers on check mode

This is supposed to be the automatic detection mode, so we should cope
gracefully with any of these being missing.

Regression introduced in commit a166517a6d2f095c96b9661e8f2519470576a513.

debian/changelog

@@ -35,6 +35,8 @@ dpkg (1.18.23) UNRELEASED; urgency=medium
       systems the header is not self-contained.
     - Handle libmd implementations built into system libc, as found on some
       BSD systems.
+    - Do not fail on missing compression libraries or headers on automatic
+      detection mode. Regression introduced in dpkg 1.18.14.
 
  -- Guillem Jover <guillem@debian.org>  Wed, 01 Feb 2017 06:28:28 +0100
 

m4/dpkg-libs.m4

@@ -59,7 +59,9 @@ AC_DEFUN([DPKG_WITH_COMPRESS_LIB], [
         ])
         AS_TR_CPP($1)_LIBS="${AS_TR_CPP($1)_LIBS:+$AS_TR_CPP($1)_LIBS }$dpkg_$1_libs"
       ], [
-        AC_MSG_FAILURE([lib$1 library or header not found])
+        AS_IF([test "x$with_lib$1" != "xcheck"], [
+          AC_MSG_FAILURE([lib$1 library or header not found])
+        ])
       ])
     ])
   ])