Slow cmake glob checks #46

On a cmake project that makes use of boost via add_subdirectory, the "Re-checking globbed directories..." stage at the start of the build will occasionally take several tens of seconds. This traces down to the use of CONFIGURE_DEPENDS on Boost.Assert and other Boost libraries.

Consider disabling CONFIGURE_DEPENDS when Boost is used as a dependency, as glob results shouldn't change and re-checking can be costly.