Replaced macro with constants

The following Merge Request (MR) has been forwarded from GitHub in order to prevent the GNOME Project from losing contributions coming from un-official channels. And for contributors to not see their valuable contributions not being accounted for.

Relevant information:

Github handle: vorot93

MR URL: https://github.com/GNOME/gnome-system-monitor/pull/6

Patch URL: https://github.com/GNOME/gnome-system-monitor/pull/6.patch

Body of the MR: