do not warn for strncpy

strncpy use in iscsi_reconnect is just fine.  Do not warn for it, and
also do not warn if clang does not recognize the flag.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure.ac

@@ -29,6 +29,7 @@ AC_ARG_ENABLE([werror], [AS_HELP_STRING([--disable-werror],
 
 if test "$ac_cv_prog_gcc" = yes; then
    WARN_CFLAGS="-Wall -W -Wshadow -Wstrict-prototypes -Wpointer-arith -Wcast-align -Wno-strict-aliasing"
+   WARN_CFLAGS="$WARN_CFLAGS -Wno-unknown-warning-option -Wno-stringop-truncation"
    if test "x$enable_werror" != "xno"; then
        WARN_CFLAGS="$WARN_CFLAGS -Werror"
    fi