Fix build on zlib-less environments

Commit 4d57e838 added support for getting I/O errors out of zlib,
but it introduced a portability problem for systems without zlib.
Repair by wrapping the zlib call inside #ifdef and restore the original
code in the other branch.

This serves to illustrate the inadequacy of the zlib abstraction in
pg_backup_archiver: there is no way to call gzerror() in that
abstraction.  This means that the several places that call GZREAD and
GZWRITE are currently doing error reporting wrongly, but ENOTIME to get
it fixed before next week's release set.

Backpatch to 9.4, like the commit that introduced the problem.

src/bin/pg_dump/pg_backup_tar.c

@@ -556,12 +556,18 @@ _tarReadRaw(ArchiveHandle *AH, void *buf, size_t len, TAR_MEMBER *th, FILE *fh)
 				res = GZREAD(&((char *) buf)[used], 1, len, th->zFH);
 				if (res != len && !GZEOF(th->zFH))
 				{
+#ifdef HAVE_LIBZ
 					int		errnum;
 					const char *errmsg = gzerror(th->zFH, &errnum);
 
 					exit_horribly(modulename,
 								  "could not read from input file: %s\n",
 								  errnum == Z_ERRNO ? strerror(errno) : errmsg);
+#else
+					exit_horribly(modulename,
+								  "could not read from input file: %s\n",
+								  strerror(errno));
+#endif
 				}
 			}
 			else