The upstream project intends the include files to be installed in
`$includedir/nlohmann/json.hpp`,
e.g. `/usr/include/nlohmann/json.hpp`. Debian did not follow that
recommendation and installed them directly in `$includedir`. Therefore
`configure` was only looking for the latter, not the former.
However, Debian will soon follow suit. Therefore detection has been
extended to check for the intended location first.
Fixes#2048.
