FindQalculate: Use FindPkgConfig instead of UsePkgConfig
Review Request #123398 - Created April 17, 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 cantor links to qualculate and cln.
Please, send it directly for master branch and remember to use the https://techbase.kde.org/Policies/Commit_Policy#Special_keywords_in_GIT_and_SVN_log_messages in commit message.