set preprocessor defines for boost system to be header only
set preprocessor defines for boost system to be header only

We get linking problems with boost system if the C++ versions
don't match (i.e. the one used to compile boost, and the current one),
specifically with boost::system::detail::generic_category_instance

Setting these preprocessor variables forces Boost to use inline variables, as opposed to
the instantiated variables in the library

Fixes https://github.com/SyneRBI/SIRF-SuperBuild/issues/161

CCPPETMR/SIRF-SuperBuild,CCPPETMR/SIRF-SuperBuild