gplugin/gplugin

Fix the build when ran in a meson subproject.
feature/meson-subproject-fix
2019-08-12, Gary Kramlich
923cbd2a8055
Fix the build when ran in a meson subproject.

We were using meson.build_root() when we should have been using
meson.current_build_dir().
The following individuals have contributed to this project.
Ankit Vani <a@nevitus.org>
EionRobb <eion@robbmob.com>
Elliott Sales de Andrade <quantum.analyst@gmail.com>
Gary Kramlich <grim@reaperworld.com>
Jason Scherer <schereja@gmail.com>
John Bailey <rekkanoryo@rekkanoryo.org>
Mike Ruprecht <cmaiku@gmail.com>
Olaf Hering <olaf@aepfle.de>
Richard Laager <rlaager@pidgin.im>