[6.0] 96e9e4500 Add -Wno-unknown-warning-option to libvgz cocktail, to deal with older clangs.

[Dridi Boukelmoune]

commit 96e9e450027a1f875cbd31be32a3d5aa14cea332
Author: Poul-Henning Kamp <phk at FreeBSD.org>
Date:   Mon Jan 30 12:07:29 2023 +0000

    Add -Wno-unknown-warning-option to libvgz cocktail, to deal with older clangs.

diff --git a/configure.ac b/configure.ac
index 3baa78565..932691e65 100644
--- a/configure.ac
+++ b/configure.ac
@@ -245,7 +245,7 @@ AC_CHECK_FUNCS([pthread_mutex_isowned_np])
 LIBS="${save_LIBS}"
 
 # zlib-specific flags
-libvgz_extra_cflags="-Wno-error=strict-prototypes"
+libvgz_extra_cflags="-Wno-error=strict-prototypes -Wno-unknown-warning-option"
 
 dnl https://github.com/madler/zlib/issues/633
 libvgz_extra_cflags="$libvgz_extra_cflags -Wno-error=deprecated-non-prototype"