do not force the use of static boost libraries if the ENABLE_SHARED variable is undefined

we only want to force the use of shared libraries if ENABLE_SHARED is
set to true. if it is unset, we don't care.
add BOOST_TEST_DYN_LINK define if shared libraries are probably not going to be used

regardless of how this is determined
Merge pull request #70 from andlaus/disable-forced-static-boost-by-default

do not force the use of static boost libraries if the ENABLE_SHARED variable is undefined