Do not error about missing zstd unless --with-zstd.

	PR lto/94259
	* configure.ac: Report error only when --with-zstd
	is used.
	* configure: Regenerate.

gcc/ChangeLog

+2020-03-25  Martin Liska  <mliska@suse.cz>
+
+	PR lto/94259
+	* configure.ac: Report error only when --with-zstd
+	is used.
+	* configure: Regenerate.
+
 2020-03-25  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/94308

gcc/configure

@@ -10019,7 +10019,7 @@ if test $gcc_cv_header_zstd_h = yes; then
 
 $as_echo "#define HAVE_ZSTD_H 1" >>confdefs.h
 
-else
+elif test "x$with_zstd" != x; then
     as_fn_error $? "Unable to find zstd.h.  See config.log for details." "$LINENO" 5
 fi
 

gcc/configure.ac

@@ -1371,7 +1371,7 @@ AC_MSG_RESULT($gcc_cv_header_zstd_h)
 if test $gcc_cv_header_zstd_h = yes; then
   AC_DEFINE(HAVE_ZSTD_H, 1,
 	[Define if you have a working <zstd.h> header file.])
-else
+elif test "x$with_zstd" != x; then
     as_fn_error $? "Unable to find zstd.h.  See config.log for details." "$LINENO" 5
 fi