meson: add systemd/elogind backends only if enabled

Right now, we are adding systemd and elogind backends to the build
system as soon as their libraries are found on the build system.
Instead, we should only add them if the libraries have been found _and_
the user has actually requested them to be included.

backend/meson.build

@@ -33,11 +33,11 @@ else
 	backend_files += files('session/direct.c')
 endif
 
-if systemd.found()
+if systemd.found() and get_option('enable_systemd')
 	backend_files += files('session/logind.c')
 endif
 
-if elogind.found()
+if elogind.found() and get_option('enable_elogind')
 	backend_files += files('session/logind.c')
 endif