Utilize the building way used in other places like the 'Release Howto' and the INSTALL file

With that method, the "krusader-build" folder way is used, as it's also followed in:

• The Release Howto: https://phabricator.kde.org/w/krusader/release-howto/
• The INSTALL file: https://commits.kde.org/krusader?path=INSTALL
• The method that Davide Gianforte wrote in the "krusader-devel" mailing list.

Differential Revision: https://phabricator.kde.org/D27534