This has the advantage of returning libraries in their load order, which
is more useful than the unspecified order that was used before.
It also means that this function now has a chance of working under other
systems such as FreeBSD, which also provides dl_iterate_phdr().
Remove space between WX_ARG_ENABLE and the opening parenthesis, this is
not allowed in m4 syntax.
Also give an error, not a warning, if CEF header is not found as it
should exist if this option was explicitly specified.
