Autoconf 2.68 is more picky about correct macro usage and thus I fixed
some wrong call conventions for AC_LANG_PROGRAM. Also factored an m4
conditional construct out from AC_INIT to avoid the "not a literal"
warning.
rl_catch_signals.
* libcurl.m4: Don't require curl-config to be present since it seems some
binary packages don't include it. Allow users to specify LIBCURL and
LIBCURL_INCLUDES for us.