[th/meson-prefix-dir] build/meson: honor prefix for udev_dir and don't use pkg-config
When building with mesond -Dprefix=/tmp/nm, then we would expect
that udev files are installed there (wouldn't we?).
The user can already explicitly set "-Dudev_dir=", or even disable installing the files with "-Dudev_dir=no".
Note that meson be default pre-populates get_option("prefix"), so there
is always something set. So we cannot just act on whether the user set a
prefix. It seems to default to /usr/local.
Note that package builds from Fedora spec file pass "-Dprefix=/usr".
I think we should honor the prefix. However, then it seems wrong to also
honor pkg-config at the same time.
In particular, because pkg-config --variable=udevdir udev gives /usr/lib/udev.
That means, if we would just prepend the default prefix "/usr" or "/usr/local"
to "/usr/lib/udev" we get the wrong result.
Note that we already to the same for autotools.