I've take that from an old doc, but I assume the flag is still the same.
"If you are linking to a dynamic runtime, then you can choose to link to
either a static or a dynamic Boost library, the default is to do a
static
link. You can alter this for a specific library
whatever by
defining BOOST_WHATEVER_DYN_LINK to force Boost library
whatever
to be linked dynamically. Alternatively you can force all Boost
libraries to dynamic link by defining BOOST_ALL_DYN_LINK."
from:
http://www.boost.org/doc/libs/1_33_1/more/getting_started.html