ITS#9307: Make --enable-<option>=mod require --enable-modules

Regression introduced by !46 (merged)/ITS#8224: configure fails with an unhelpful error message when configuring with --enable-<option>=mod but not --enable-modules.

This MR provides a more specific message in this case, makes it an error for both backends and overlays, and removes the automatic fallback to static.

Alternate approach would be to simply fix the regression and restore the RE24 behaviour. I am fine with either; feedback is welcome.

RE24:

./configure --enable-backends=mod
[...]
configure: WARNING: building static bdb backend

RE25:

./configure --enable-backends=mod
[...]
checking configure arguments... configure: error: slapd requires a backend

this branch:

./configure --enable-backends=mod
[...]
checking configure arguments... configure: error: --enable-backends=mod requires --enable-modules