Similar to 5a5229b4: use more C++14
constexpr.

fails on it. This fixes.

Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Thanks to C++14, we can declare and fill variables inside `constexpr`
functions.  This means me can stop make assumptions on the `struct`
layouts without losing `constexpr`.

Closes #393

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!

The new implementation is copied from another project and is
BSD-licensed.  It is exception-safe and can parse IPv6 scope ids with
interface names.

first call fchmod() to prevent TOCTTOU, then apply permissions using
chmod()

Fixes the NFS client build, closes #305.