[4283] Made boost at least includes (vs libraries)

INSTALL

@@ -5,9 +5,9 @@ To then build from source:
     ./configure
     make
 
-Kea depends on C++ compiler, make, libtool, boost libraries, log4cplus
-and one crypto library (either OpenSSL or Botan) for compilation.
-Optional backends (MySQL, PostgreSQL and Cassandra) have
+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.