Changed Makefile to compile on newer Linux systems with a consolidate…

[mracter]

…d libsystemd package (ie. Ubuntu 16.04).

This fixes the following issue encountered at compile-time:

Package libsystemd-login was not found in the pkg-config search path.
Perhaps you should add the directory containing `libsystemd-login.pc'
to the PKG_CONFIG_PATH environment variable
No package 'libsystemd-login' found
In file included from functions.c:23:0:
lightum.h:2:21: fatal error: gio/gio.h: No such file or directory
compilation terminated.
<builtin>: recipe for target 'functions.o' failed
make: *** [functions.o] Error 1

Recommend new branch for this change.