[boost] gcc visibility support

Subject: [boost] gcc visibility support
From: Vladimir Prus (ghost_at_[hidden])
Date: 2009-06-07 11:17:12

Over in issue 2114 (https://svn.boost.org/trac/boost/ticket/2114), Jürgen has
provided a patch to add gcc visiblity support to all separately compiled libraries.
gcc visibility is a mechanism similar to dllexport on Windows, with the same
benefits.

I have tried to apply it, and rebuild program_options. The the size of release 
shared library went from 272K down to 224K. The number of relocations went
from 972 to 872. Both changes seem to be good.

It seems to be unlikely to get the authors of every compiled library to
look at the patch, so it looks like an alternative approach is necessary.
Does anybody have any objections to this approach? If not, I'll check
this in somewhere before Jule 15 deadline, and will take care of checking
regression results afterwards.

- Volodya