Add support for elogind

[oz123]

Uses similar patch to the one already included in mate-screensaver.

[lukefromdc]

I have systemd not elogind on Debian Unstable (Sid) but this configured fine and built fine. Config messages worked, showing

Configure summary:

        mate-system-monitor 1.27.0
        ==========================

        Source code location:   .
        C Compiler:             gcc
        CFLAGS:                 
        WARN_CFLAGS:            -Wall -Wmissing-prototypes
        C++ Compiler:           g++
        CXXFLAGS:               
        Elogind support:        no
        WARN_CXXFLAGS:          -Wall -Woverloaded-virtual
        Maintainer mode:        no
        Systemd support:        yes
        wnck support:           yes

Note that we also support building on meson and this only changes the options in autotools.

[oz123]

@lukefromdc I am not so familiar with meson. Happy to get some guidance here. I myself don't know enough about it to teach it either

[cwendling]

I cannot test this because I'm not gonna install libelogind which has serious consequences on other packages here and I don't wanna deal with it. I could try in a VM, but call me lazy for now.

Anyway, I don't think this PR works as-is, even with libelogind installed I doubt the linking phase would work, as the library wouldn't be used on the link command. If it does work, please explain how comes.

[lukefromdc]

I myself don't know enough about meson to help with that, I can build with it and test packages but have not yet written or substantially modified meson configuration files.

[oz123]

@cwendling I refactored my patch according to your suggestions.
I also tested it on ubuntu vagrant machine which has no libelogind and it seems to behave as desired:

vagrant@m1:~/mate-system-monitor$ ./configure --enable-elogind 
checking for a BSD-compatible install... /usr/bin/install -c
checking whether build environment is sane... yes
....
checking how to hardcode library paths into programs... immediate
checking for
    glib-2.0 >= 2.56.0
    libgtop-2.0 >= 2.37.2
    gtk+-3.0 >= 3.22.0
    gtkmm-3.0 >= 3.8.1
    libxml-2.0 >= 2.0
    librsvg-2.0 >= 2.35
    glibmm-2.4 >= 2.22
    giomm-2.4 >= 2.26.0
    gmodule-2.0
... yes
checking for glib-2.0 >= 2.56.0... yes
checking for libelogind... no
configure: error: *** elogind support requested but libraries not found
vagrant@m1:~/mate-system-monitor$ 

[lukefromdc]

With no landline I cannot download distro installers to test. If I did they'd go on extra drives as I also have no VM experience. Thus I have no way to test any of the non-systemd stuff