build a shared library on Linux

I'm using these two patches to build a shared library in [my Fedora package of linenoise](https://bugzilla.redhat.com/show_bug.cgi?id=892988).

This is a much less intrusive change than some of the other build patches floating around (autotools, waf, CMake...) but it's also less portable. It will probably only work with gcc >= 4.0 and GNU make. So if you aren't interested in accepting it I don't mind :-)