Makefile.in: make sure we actually compile http_parser.c
Tell Makefile.in how to compile .cpp files

Apparently, there isn't a rule for that in BSD make.
Makefile.in: add explicit rule for http_parser.o

It turns out that, without an explicit rule, BSD make puts the
object file into the toplevel directory.

On the contrary, gmake puts the object file in http-parser/.

So, add an explicit rule to force the object file into the toplevel
directory of libneubot, and make the linker happy.
configure: Add OpenBSD-specific rules

Last time I checked, my code worked with both libevent 1.4 and
libevent 2.0, therefore, there was no need to add compiler flags
on OpenBSD because OpenBSD includes libevent 1.4 by default.

Now, however, libevent 2.0 is needed; therefore, compiler flags
to consider /usr/local are now needed as well.
echo poller utils: do not use libevent 1.4 headers

Predictably, if we mix libevent 1.4 and libevent 2.0 headers, the
compiler is not going to like it.

Always use libevent 2.0 headers, since we use libevent 2.0.
Makefile.in: more warnings
poller.c: silence warnings
Delete (and ignore) files generated by SWIG

Rationale: since I work with systems using different versions
of SWIG, I'm getting tired of having to revert the small changes
caused by such different versions all the time.
configure: make swig optional

In theory, one can build libneubot without swig (which is used
to build files that are only used by me to experiment), therefore,
do not fail if swig is not installed.
Merge branch 'build_system_luv++'