db: fix build with clang 16

The configure scripts frequently use `main` returning an implicit `int`,
which causes spurious failures when CC is clang 16+. This is fixed by
patching the provided macros and regenerating the scripts with
autoreconfHook, though it requires some manual processing (see below).

The upstream `configure.ac` is written in such a way that it requires
fixups and post-processing.

* Fixups are required because the original build process just cats the
  macros together into one file. When `aclocal` is run, it does not pick
  up all of them. This is worked around by catting the missing macros to
  a file that is picked up by autoreconfHook.
* Post-processing is required to populate the version information. This
  is done in a subshell to avoid polluting the environment with the
  contents of `RELEASE`. Otherwise, the build will fail because the
  version C macros it expects will not be defined.

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