This version of the AX_CFLAGS_WARN_ALL macro uses a far more robust
method of detecting compilers to select appropriate warning flags. It
also uses the AX_PREPEND_FLAG macro instead of AX_APPEND_FLAG.
This version of ax_cflags_warn_all.m4 (and ax_prepend_flag.m4) has been
submitted upstream; see https://savannah.gnu.org/patch/?9673 for details.
