package/polkit: needs glibc

The recent bump of polkit introduced a version that now unconditionally
uses glibc-specific functions from the getnetgrent() familly, which are
not available on musl or uClibc-ng.

Hide away polkit when the C library is not glibc.

Propagate the dependency to packages that select polkit: udisks and
systemd. For systemd, add a comment.

Fixes:
    http://autobuild.buildroot.org/results/d78/d783e6db60b187aa591f4b0150c590ab3aa5252f/ (musl)
    http://autobuild.buildroot.org/results/ba5/ba5da960c70c765bee753a79b9ad2c10967d05d6/

 (uclibc)
    [...]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Marek Belisko <marek.belisko@open-nandra.com>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Artem Panfilov <apanfilov@spectracom.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>