Add compile flags to blt_add_library and executable macros

[white238]

@cyrush says this should smooth out CUDA support. It also adds a nice feature though this could cause problems with adding compiler flags too liberally to mixed source libraries.

[davidbeckingsale]

Would they be "extra" compile flags, or the only compile flags?

[white238]

They would be the initially set flags since there wouldn't be a target created yet there wouldn't be any to override. Also any language specific flags CMake will automatically add. This wouldn't stop you from adding any to sources or to the target after the blt_add_* call.