This matches with what Debian does, and is what the gcc maintainers
suggest.
The idea is to have ldc internal headers (those that cannot be used by
gdc and cause conflicts) in /usr/lib/ldc/%{_target_platform}/include/d/,
and keep library header files (those that both ldc and gdc can use) in
/usr/include/d/.
https://bugzilla.redhat.com/show_bug.cgi?id=1781685