BUILD/MINOR: fix Lua build on Mac OS X (again)

Previous commit (13113d6 "MINOR/BUILD: fix Lua build on Mac OS X")
contains a typo, it uses "-export-dynamic" instead of "-export_dynamic"
(dash instead of underscore), despite what the commit message suggests,
and it obviously doesn't work. Thanks to Kirill A. Korinsky for reporting
it.

This patch should be backported on each version from 1.6 like the
aforementionned one above.
1 file changed