Scons: Move some library dependency declarations into library_deps.py

Scons: Move some library dependency declarations into library_deps.py

Declare dependencies of the 'sel' library in order to fix some of the
worst duplication.

Change the code in library_deps.py to take dependencies for 'sel' from
both LIBRARY_DEPENDENCIES_DEFAULT and PLATFORM_LIBRARY_DEPENDENCIES.
Replace the transitive-closure implementation in library_deps.py with
one that is simpler and easier to understand.

BUG=none
TEST=build

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=8348

[Nick Bray (chromium), 2012-04-20 21:05:45]

LGTM