Control symbol visibility for functions exported in the pg_repack
shared library, to avoid symbol conflicts with other libraries,
particularly those which may be loaded via shared_preload_libraries
and use conflicting function definitions of our common pgut code,
such as pg_statsinfo.
Verified this fix on my Debian x86_64 machine with PostgreSQL 9.4 and
pg_statsinfo 3.0.2. Would be nice to test this fix on a few other
platforms such as OS X, though I had some difficulty getting
pg_statsinfo to build on OS X. Hopefully closes Issue #43, thanks
to bwtakacy for the report and demonstration of the commited fix in
pg_reorg.
