Both AvogadroLibs and Eigen3 are optional

The REQUIRED was added during the initial port to Frameworks 5, but
the conditional code enabled by Avogadro and Eigen3 availability is
still in place, and in fact Kalzium still compiles and run without
both (minus some functionalities).

Reviewed-by: Andreas Cord-Landwehr <cordlandwehr@kde.org>