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.