FindBoost: Add suport for custom namespaces

When building boost with an alternate namespace the libraries generated
will have a different naming convention.  This is often done to ensure
no symbol conflicts with external libraries built against a different
version of boost.  If the namespace used is "myprivateboost::" instead
of "boost::" then the libraries built will be named myprivateboost_foo
instead of boost_foo.  Add an option to specify a custom namespace used
to alter the library names that get searched for.
Merge topic 'FindBoost-namespace' into next

17485e37 FindBoost: Add suport for custom namespaces