opensp: fix build with clang 16
There are two issues preventing a build with clang 16:
* The configure scripts are too old and generate invalid `config.h`.
This is fixed by using `autoreconfHook` with clang.
* Use of the `register` storage class specifier, which is not allowed in
C++17 (the default when building with clang 16). This is fixed by
removing that usage.