incremental dependencies

[v4hn]

Some configure options / dependencies only become available
in case the user selected some other option before (e.g. Xpm image support
in nethack doesn't work without libX11 support).

This does not necessarily require a change in our codebase, but we need a unified way
to handle these cases and "(select only when also XYZ)" in the description is not adequate at all.

Is something like the following supported at the moment for DEPENDS/CONFIGURE and do we want this?
@sofar mentioned that there should be no logic in CONFIGURE/DEPENDS...

if selected_dependency ABC; then
   optional_dependency EFG "--enable-EFG" "--disable-EFG" "EFG support"
fi

[cavalier38]

DEPENDS can't have logic as it would break the depends tree.

An alternative would be to support a list of modules. This can be used for both batch and incremental depends.

for batch depends:

optional_depends "module1,module2,module3" "--enable-flag" "--disable-flag" "Info"

for incremental depends:

optional_depends "incremental:base" "--enable-inclremental" "--disable-incremental" "Info"
optional_depends "base" "--enable-base" "--disable-base" "Info"

There is a different between , and : this different is mainly for formating the depends question. The implementation of , and : would probably be the same.

[v4hn]

The logic of `optional_depends "base:incremental" (which way around?) would then automatically add
base as an accepted optional dependency on "y" for the module and the second question will not be asked.

@Ratler, @sofar: what do you guys think?