To use a DESTDIR method just change the
--prefix option in the install command:
./b2 --prefix=$DESTDIR/usr install threading=multi link=shared
(where $DESTDIR is set to /some/path/to/directory)
If you wish to get verbose output from ./b2, add
-d+2 to the invocation. Doing that, you can see that only g++ is used, and that the default compile flags are -O3. If you wish to append any other CXXFLAGS, add
cxxflags="-fwhatever" to the first ./b2 invocation (the second invocation for install only copies the files).