[4283] Dependency on boost reworded slightly.

INSTALL

@@ -5,11 +5,11 @@ To then build from source:
     ./configure
     make
 
-Kea depends on C++ compiler, make, libtool, boost (at least includes),
-log4cplus and one crypto library (either OpenSSL or Botan) for
-compilation.  Optional backends (MySQL, PostgreSQL and Cassandra) have
-additional dependencies. For detailed installation directions, see the
-guide at doc/guide/kea-guide.txt or doc/guide/kea-guide.html.
+Kea depends on C++ compiler, make, libtool, boost (at least includes, but many
+OSes require boost-system library), log4cplus and one crypto library (either
+OpenSSL or Botan) for compilation.  Optional backends (MySQL, PostgreSQL and
+Cassandra) have additional dependencies. For detailed installation directions,
+see the guide at doc/guide/kea-guide.txt or doc/guide/kea-guide.html.
 
 You can find user-contributed OS-specific build/installation
 instructions on the Kea wiki: http://kea.isc.org/wiki/Install