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.

cmake/modules/AddGMPFlags.cmake

@@ -21,7 +21,7 @@ set(HAVE_GMP ${GMP_FOUND})
 if(GMP_FOUND)
   dune_register_package_flags(
     LIBRARIES GMP::gmpxx
-    COMPILE_DEFINITIONS "ENABLE_GMP=1"
+    COMPILE_DEFINITIONS "HAVE_GMP=1"
   )
 endif()
 
@@ -30,7 +30,7 @@ function(add_dune_gmp_flags _targets)
   if(GMP_FOUND)
     foreach(_target ${_targets})
       target_link_libraries(${_target} PUBLIC GMP::gmpxx)
-      target_compile_definitions(${_target} PUBLIC ENABLE_GMP=1)
+      target_compile_definitions(${_target} PUBLIC HAVE_GMP=1)
     endforeach(_target ${_targets})
   endif(GMP_FOUND)
 endfunction(add_dune_gmp_flags)