Makefile.in: add -Wunreachable-code

I was reading about the CVE-2014-1266 SSL/TLS Apple bug on
ImperialViolet and learnt that clang has a separate flag for
unreachable code, -Wunreachable-code, that is not included
in the -Wall warnings [1].

So, let's add -Wunreachable-code to Makefile.in.

[1] https://www.imperialviolet.org/2014/02/22/applebug.html