Simplify installHeader

[agagniere]

A replacement of #4 that keeps compatibility with 0.14

[torque]

looks good to me, for whatever my opinion is worth

[agagniere]

@torque So this change has a consequence I should have maybe highlighted:

previously all 3 libs would export all headers.

now, libtls only exports tls.h, and libssl exports the openssl header folder, while libcrypto doesn't export headers at all.

Because I've only ever used libssl (e.g. in libpq) it works for my use case.

But there may have been reasons to export all headers from all libs ? in particular libcrypto

[torque]

I expect most projects that use libressl are using the openssl bindings rather than the libtls API, so checking whether or not the header situation actually works for those may be less trivial to do. zig build -Dbuild-apps still appears to work on this branch, at least, though I'd need to check that that's not cheating with the headers. I'll poke it in a little bit more detail tomorrow.

[torque]

I fiddled around with a couple of other things and they appear to build ok. I think having libtls not export the openssl includes makes more sense than what was being done before. Merge at will.