meson: disable automatic shell completion on prefix missmatch

meson: disable automatic shell completion on prefix missmatch

Currently one can choose a prefix completely different from the one
bash-completion and fish use, as per their pkg-config file.

In such cases, flag a warning and disable it. People can always manually
provide the completion directory/ies to re-enable.

    $ meson setup --prefix=/tmp/example build/
    ...
    WARNING: User provided prefix '/tmp/example' differs from bash-completion one '/usr'. Disabling completion.
    ...
    WARNING: User provided prefix '/tmp/example' differs from fish one '/usr'. Disabling completion.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/209
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>