Rewrite FindQuadMath to provide imported targets

Summary

This MR adds imported-targets to the FindQuadMath.cmake module, following modern cmake guidelines.

Details

Instead of exporting include directories and libraries as variables, the find modules exports an imported target that contains all the necessary information to include and link QuadMath. The imported target is named: QuadMath::QuadMath.

Acknowledgements

This find-module is developed together with @lukas.riedel