doing magic with package dependencies (?)

(This message is the second of a two-parter; in the first
part, I commented that we might like to compile packages
with very explicit dependencies, e.g.

   gcc -I/.-ark-deploy/zlib--1.1.3/include \
       -I/.-ark-deploy/libpng--1.0.9/include \
       -L/.-ark-deploy/zlib--1.1.3/lib \
       -L/.-ark-deploy/libpng--1.0.9/lib ...
)

Now, it would be More Than a Little Tedious to spell out all
such dependencies... However, note that we already say
things like...

<configure>
  <param name="configure_args">--gung-ho --with-database=berkdb --with-jpeg --with-png --with-xpm --without-mule --without-dragndrop --without-cde --without-ldap</param>

  <constraint><dependency type="normal" name="db" /></constraint>
  <constraint><dependency type="normal" name="jpeg" /></constraint>
  <constraint><dependency type="normal" name="libpng" /></constraint>
  <constraint><dependency type="normal" name="xpm" /></constraint>
</configure>

Those <dependency .../> constraints already specify the
dependencies! -- all we need to do is have them magically
turn into -I... and -L... flags :-)

So, howzabout:

* The ARK engine "remembers" dependent packages that it
  bumps into while satisfying <constraint>s ...

* The engine provides a (Python) interface that lets us
  query this information

* It's then a simple matter to write

  <param name="DEP_INCS"> !dependentPkgs(format='-I') </param>
  <param name="DEP_LIBS"> !dependentPkgs(format='-L') </param>
  <param name="CFLAGS"> -O2 @param:DEP_INCS@ @param:DEP_LIBS@ </param>

* The package state will then record the exact dependencies
  (in its records of the values of DEP_INCS and DEP_LIBS).

  (If we wanted to be more explicit about it, we could do
  something like

    touch /d/ark-state/package/zlib--1.1.3/depends:xemacs--21.4.4
  )

Any other {weird thoughts,alarm bells} occur to you all?

Will