For releasing we want to link qdoc statically to libclang and ship
a "fat" binary, which allows us to not distribute separate libclang.
In the same time our users may want to link qdoc to an existing
libclang binary. Both cases make sense, especially that the static
build required some invasive patches in libclang.

Partially reverts fddf7ee5



Task-number: QTBUG-68178
Change-Id: Iaeab59c40b881907f0fd5f3ccefb6700dcd2dded
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>