Starting with autoconf 2.70, obsolescence warnings are output by
default. To keep the script as compatible as possible, I build
autoconf 2.60 and ran the autoupdate that came with it on
configure.ac and all m4 files.
Extra compile options are detected at configure time with new
XIPH_ADD_*FLAGS m4 macros.
Options enabled include -Wextra, -D_FORTIFY_SOURCE=2 and stack
protection.