travis: only install required apt packages

[MarcelRaad]

This allows to selectively update some clang jobs to xenial as the LLVM repository name is different for xenial than for trusty. Also, it saves a little time for most Linux jobs.

[danielgustafsson]

Is it possible to also only compile dependencies such as MesaLink for the mesalink jobs? This is currently done in before_script but my Travis skills aren't up to scratch for figuring out if that can be made job dependent.

[MarcelRaad]

I wondered about that too. Probably @bagder had a good reason to do that starting in 277d3cd for mbedtls (and then for wolfssl and mesalink) as the prior brotli and BoringSSL builds were only executed for those jobs?

[MarcelRaad]

FYI: I hope we can get rid of some of those manually built dependencies. I have another branch based on this one to update some jobs to Xenial (not all though because Xenial's Valgrind doesn't like Xenial's OpenSSL, which is why this PR is needed). Xenial contains some of the dependencies missing in Trusty (but at least wolfSSL's version is too old for test 311).

[bagder]

@bagder had a good reason to do that starting in 277d3cd for mbedtls (and then for wolfssl and mesalink)

Who really knows how travis works? I certainly don't and mostly try every possible combination until it seems to behave... My intentions were that the builds of those dependencies should end up in directories that are cached between invokes so that only 'make install' would be necessary for most builds when the cache is used. We can of course polish the logic so that we only 'make install' dependencies that are actually used.