oss-fuzz check CXXFLAGS as well for -fsanitize= foo

Change-Id: Ifd4aa349a9debcee4b8446c2e6e84ccc36a84efd

configure.ac

@@ -6526,7 +6526,7 @@ _ACEOF
             dnl moot, though):
             my_linkflagsnoundefs=$LINKFLAGSNOUNDEFS
             if test "$COM_IS_CLANG" = TRUE; then
-                for i in $CXX; do
+                for i in $CXX $CXXFLAGS; do
                     case $i in
                     -fsanitize=*)
                         my_linkflagsnoundefs=