Install headers for the vendored libuv

[aantron]

See https://dune.readthedocs.io/en/stable/foreign-code.html?highlight=install%20headers#installing-header-files.

cc @ulrikstrid

[aantron]

The linked commit installs all the libuv headers into Luv's installation directory. It is enough to have (libraries luv) to be able to use #include <uv.h>.

There is a generator for the installation stanzas on the Luv side, which is integrated into the libuv upgrade script.

The commit also adds a test consisting of a toy project that depends on Luv and does #include <uv.h>. I set it up to run in Travis. I tested it locally with uv.h renamed to uvv.h to prevent the test from falsely succeeding by picking up a system uv.h. I opened #84 to later check if Travis has any system uv.h installed, and sabotage it during the test, if it is present.