So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Apparently, Travis-CI will never upgrade its ancient Ubuntu Trusty,
so I have to get Boost from somewhere else.

Thanks to Martin Hierholzer for his PPA.

Also test the newest available GCC version.

After commit a72d1200, the Travis-CI
build fails, because we have been using GCC 5 there...  so let's
switch to GCC 6 to test the oldest possible GCC version.

The clang/libc++ version in the default xcode8 image is unable to
compile MPD due to incomplete C++14 support.

Prepare for Mac OS X support by omitting those Ubuntu-specific
commands.

.. and not update-alternatives, which requires "sudo".