Update openspecfun and install it properly

This follows #6001: since we now install openlibm correclty,
we can update openspecfun to current master, which uses
headers from $(includedir).

My goal is to check that everything works fine, and if so
it would be nice if you would release new tarballs for openlibm
and openspecfun, for my RPM packages.

I mistakingly committed a small fix to openspecfun instead of
pushing my branch for review. Hopefully it's not too controversial:
https://github.com/JuliaLang/openspecfun/commit/5237418f1fc9d244dbbf7a617d149f2abbfd5bc6

@ihnorton I've followed what you did for openlibm in af2905e (cf. #6063).
Can you confirm it works on Windows?

Finally, I've added a commit to clean .dylib, as requested the last
time.