[6.0] 505373add Fix libvgz build error on gcc7

[Dridi Boukelmoune]

commit 505373add3a01e40a256c1223c048e1c0f6f3268
Author: Dag Haavi Finstad <daghf at varnish-software.com>
Date:   Mon Sep 24 13:27:36 2018 +0200

    Fix libvgz build error on gcc7
    
    This adds -Wno-implicit-fallthrough for libvgz to satisfy gcc7.
    
    The -Wno-unknown-warning-option is there to satisfy older gcc/clang that
    don't recognize -Wno-implicit-fallthrough.

diff --git a/configure.ac b/configure.ac
index 0c0266aae..00172c7cc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -261,6 +261,11 @@ if test "$ac_cv_have_viz" = no; then
 fi
 CFLAGS="${save_CFLAGS}"
 
+if test "x$GCC" = "xyes"; then
+	libvgz_extra_cflags="${libvgz_extra_cflags} -Wno-unknown-warning-option -Wno-implicit-fallthrough"
+	AC_SUBST(libvgz_extra_cflags)
+fi
+
 SAN_CFLAGS=
 SAN_LDFLAGS=
 UBSAN_CFLAGS=