libreadline requires libtermcap on OpenBSD.

"rake all" fails on OpenBSD without the patch:

```
...
LD    build/host/bin/mirb 
...
/usr/lib/libreadline.so.3.0: undefined reference to `tgetnum'
/usr/lib/libreadline.so.3.0: undefined reference to `tgoto'
/usr/lib/libreadline.so.3.0: undefined reference to `tgetflag'
/usr/lib/libreadline.so.3.0: undefined reference to `tputs'
/usr/lib/libreadline.so.3.0: undefined reference to `tgetent'
/usr/lib/libreadline.so.3.0: undefined reference to `tgetstr'
collect2: ld returned 1 exit status
```