FindQalculate: Use FindPkgConfig instead of UsePkgConfig
Review Request #123557 - Created April 29, 2015 and submitted
The latter is deprecated and doesn't respect the PKG_CONFIG environment variable which makes it easy to deal with differently named pkg-config executables, e.g. arch-prefixed ones.
Configured, built and checked that plasma-workspace links to qualculate and cln.