OpenBSD support

This patchset fixes several issues that didn't allow to build `telegram` on OpenBSD:

* support for GCC <= 4.4,
* newer `ax_lua.m4` ([patched for OpenBSD](https://savannah.gnu.org/patch/?8318), includes a [patch for FreeBSD](http://git.savannah.gnu.org/cgit/autoconf-archive.git/commit/?id=0caa42ca6cbbbf9e18821d824e7a1422c0a6fb8e)),
* updated `README.md` with instructions for OpenBSD,
* updated `.gitignore` to ignore generated files.