Upstream diff to fix missing declarations with --without-zlib:
* image.c:1028:34: error: 'gzip_data_pipe' undeclared here (not in a function)
*    { decompressors+1, "gzip -dc", gzip_data_pipe, { "z", "gz", "gzip", NULL } },
*                                   ^~~~~~~~~~~~~~
See also: https://bugs.gentoo.org/show_bug.cgi?id=596414

--- a/src/lib/shade/image.c
+++ b/src/lib/shade/image.c
@@ -36,6 +36,8 @@
 
 #ifndef HAVE_LIBZ
 # define HAVE_LIBZ 0
+#elif HAVE_LIBZ
+# define HAVE_LIBZ 1
 #endif
 
 #if HAVE_LIBZ
@@ -1025,9 +1027,15 @@
 };
 
 static struct filter decompressors[] = {
+#if HAVE_LIBZ
   { decompressors+1, "gzip -dc", gzip_data_pipe, { "z", "gz", "gzip", NULL } },
-  { decompressors+2, "bzip2 -dc", NULL, { "bz2", "bzip2", NULL } },
+#else
+  { decompressors+1, "gzip -dc", NULL, { "z", "gz", "gzip", NULL } },
+#endif
+  { HAVE_LIBZ ? decompressors+2 : NULL, "bzip2 -dc", NULL, { "bz2", "bzip2", NULL } },
+#if HAVE_LIBZ  
   { NULL, NULL, zlib_data_pipe, { "zlib", NULL } },
+#endif
 };
 
 static struct filter converters[] = {