community/mate-{screensaver,session-manager}: build with elogind support

In MATE 1.24 a elogind compile option was introduced to mate-session-manager and mate-screensaver [1].

When installing packages elogind and polkit-elogind, among others WiFi can be turned on/off with media keys and a suspend button becomes available in the shutdown dialog.

If elogind isn't installed, this commit has no impact on the behaviour.

Reference to compile option in MATE source [2][3] and how Gentoo is setting it up [4][5].

Cc maintainers: @fcolista @alacerda

• [1] https://mate-desktop.org/blog/2020-02-10-mate-1-24-released
• [2] https://github.com/mate-desktop/mate-screensaver/blob/1.26/configure.ac#L950-L974
• [3] https://github.com/mate-desktop/mate-session-manager/blob/1.26/configure.ac#L128-L152
• [4] https://gitweb.gentoo.org/repo/gentoo.git/tree/mate-extra/mate-screensaver/mate-screensaver-1.26.1.ebuild#n63
• [5] https://gitweb.gentoo.org/repo/gentoo.git/tree/mate-base/mate-session-manager/mate-session-manager-1.26.0.ebuild#n66