boost: fix build problem by always building a threaded version

boost: fix build problem by always building a threaded version

Boost normally allows to build a non-threaded variant by passing
threading=single or a multi-threaded variant by passing
threading=multi.

Unfortunately, the build of threading=single doesn't seem to work any
more, due to bizarre things in the build system. We get "duplicate
target" errors, that according to
http://lists.boost.org/boost-build/2012/11/26582.php should appear if
we ask for both threading=single,multi. But it seems to happen even in
the threading=single case.

Since Boost is such a big C++ beast, it probably doesn't make much
sense to try to support it on toolchains that don't have thread
support. So, we make the boost package depend on thread support. If
someone cares enough in getting Boost to work in a non-threaded
environment, then we can always revert back.

Note that the boost package has no reverse dependencies in Buildroot,
so we don't need to propagate this new dependency anywhere.

Fixes:

  http://autobuild.buildroot.org/results/439e72ac74c8058f30977e6abc39acd6379a17d3/build-end.log

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>