Improve CMake handling of GNOME libraries
Summary:
GNOME libraries were being handled in a manner not
as elegant as possible with CMake's built-in pkgconf handling.
This patch uses CMake's built-in pkgconf handling to trim excess
code and improve style (${GTK_LIBRARIES} -> PkgConfig::GTK+3).
Test Plan: See that everything still builds and links as expected.
Reviewers: Plasma, gikari, apol
Reviewed By: gikari, apol
Subscribers: apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D28086