build: Make DBus code generation more portable

The $'...' quoting syntax is a Bash extension and might not be
available in other Bourne Shell compatible shells:
https://www.gnu.org/software/bash/manual/html_node/Major-Differences-From-The-Bourne-Shell.html#Major-Differences-From-The-Bourne-Shell

https://bugzilla.gnome.org/show_bug.cgi?id=734630