Added import/export macro's for STEPattribute operator==, operator!= and sameADesc().
Removed pure virtual operator= reference from SDAI_Select class.
(Results in linking errors on windows otherwise)
Added SC_SCHEMA_EXPORT macro for each generated entity class.
Added DEFINE_DLL_IMPORTS to add_schema_dependent_test macro.
Added stepcore to libraries of aggregate_bound_runtime test to fix linker error.
Merge pull request #280 from stepcode/review/msvc_linker_fixes

Review/msvc linker fixes