Add two new warnings to warning.m4

I recently learned that GCC has improved -Wdangling-reference and
added -Wunterminated-string-initialization.  Both of these seem
sensible to me, so this patch adds them to warning.m4.

gdb rebuilds cleanly with this in place on x86-64 Fedora 43.

Reviewed-By: Tom de Vries <tdevries@suse.de>