ENH: Allow use of Boost 1.49.

Tested with Boost 1.49.  "find_package(Boost VERSION ..." allows
VERSION or higher, so 1.51 will configure, too.

Change-Id: Ib982752d7d2da8cb614d9f965bf721f3bd1d3d87
Merge topic 'BoostCLIWarnings'

b933731 ENH: Allow use of Boost 1.49.