glibc: depend on a static kernel version for --enable-kernel

Instead of moving with kernel-headers we keep glibc to 2.6.32 which
seem to be a common standard for most distros. The former sometimes
cause some issues if you aren't careful which even could result
in your box not booting.
kernel-headers: bump headers to version 3.16.1
linux: version bumped to 3.16.1
systemd: version bumped to 215

This bump has been long overdue. The compat patch is necessary to maintain
semi compatibility with some applications that yet has adapted to the combined
libsystemd without actually installing the compat libs.

We might have to fix a couple of modules to behave that depend on systemd, but
so far the only one I've found that need a small fix is sane-backends. Eventually
we should be able to drop the compat patch that only install a couple of .pc files.
dbus: Cleanup build and enable systemd support

Ugly hack using module_installed systemd to avoid cyclic dependency.
systemd: output format changed, fixed systemd.plugin
Merge pull request #676 from Ratler/systemd-bump

systemd bump including some other required changes for the upgrade.