Handle optional dependencies GMP, QuadMath, and Vc by directly passing HAVE_xxx flags

Summary

This MR changes the way optional dependencies are enabled in cmake, following the MR !1294 (merged). Essentially the add_dune_xy_flags function and the dune_register_package_flags now sets HAVE_XY directory instead of writing a config.h entry and setting ENABLE_XY afterwards.

In this MR the package flags for GMP, QuadMath, and Vc are updated.

added buildsystem label

changed milestone to %CMake Modernization

While I am in favor of this, I think we need to make at least one changelog entry on the removal of all ENABLE_*=1. I was using this some time without knowing that I shouldn't have.

approved this merge request

added 1 commit

• b91d9cfa - Add a changelog entry for all ENABLE_XY flag changes

Compare with previous version

enabled an automatic merge when the pipeline for b91d9cfa succeeds

merged

mentioned in commit 76784f35