bdw-gc includes in libguile.h

[Andy Wingo]

Hello,

I think we made a mistake in exposing bdw-gc.h to libguile.h users.
gc.h is quite scrupulous to not include it, but smob.h, inline.h
(sometimes), and pthread-threads.h pull it in.

Besides the modularity concerns that lead us to need to add bdw-gc libs
and cflags to Guile's libs and cflags, there is an acute problem, and
that is that we enable pthread redirects -- so users of libguile get
pthread_create et al re-#defined.

I think that in 2.2 we should not expose libgc interfaces in libguile,
and that in 2.0 we should disable pthread redirects.

Andy
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