Download openwrt from git and do a trial build of it for your platform. You will need various libaries and compilers built. git://git.openwrt.org/openwrt.git
The script packaging/openwrt/build-unstrung.sh contains a simple way to build the code "in-tree" --- that is without creating a tar-ball or a git commit, etc. It contains:
make package/unstrung/prepare USE_SOURCE_DIR=/ssw/projects/pandora/unstrung V=s
make package/unstrung/compile USE_SOURCE_DIR=/ssw/projects/pandora/unstrung V=s
make package/unstrung/install USE_SOURCE_DIR=/ssw/projects/pandora/unstrung V=s
The script will need adjusting as it has the USE_SOURCE_DIR hard coded to the directory where unstrung has been checked out.
To build, you will need to have libboost installed, which is CONFIG_boost-libs-all=y
As boost is just libraries (mostly in the form of C++ templates), there is no target size impact of just building it all. In menuconfig, boost is located at: Libraries->boost
The result will be an ipkg file: ls -l bin/ramips/packages/base/unstrung_1.00-1_ramips_24kec.ipk
http://wiki.openwrt.org/doc/devel/packages
http://wiki.openwrt.org/doc/howto/build