CMake generated config defines can cause tedious rebuilds

[biddisco]

I am frustrated by the hpx_add_config_define macro that generates a variable that is in turn added to config/defines.hpp - when changing a parcelport parameter, modifying this file triggers a complete rebuild of the entire hpx stack, which is of course not necessary.

I plan to modify the config macros to allow a namespace type var so that we can put verbs/parcelport variables into their own config/namespace_defines.hpp etc. current behaviour will be left unchanged for variables specified without an optional namespace param, so that variables can be incrementally moved into separate config/xxx_defines.hpp files