Allow config #defines to be written out to custom config/defines.hpp …

[biddisco]

The existing cmake macro can be used to generate #defines for the global
hpx/config/defines.hpp file by using

  hpx_add_config_define(HPX_HAVE_LOGGING)
  hpx_add_config_define(HPX_HAVE_MAX_CPU_COUNT ${HPX_WITH_MAX_CPU_COUNT})
  ...

and then written to hpx/config/defines.hpp using a new macro and
specifying the default namespace

write_config_defines_file(
  TEMPLATE "${PROJECT_SOURCE_DIR}/cmake/templates/config_defines.hpp.in"
  NAMESPACE default
  FILENAME "${CMAKE_BINARY_DIR}/hpx/config/defines.hpp")

This keeps the existing behaviour unchanged.
However a new NAMESPACE feature is supported to allow the user to add

  hpx_add_config_define_namespace(
    DEFINE    HPX_PARCELPORT_VERBS_DEVICE
    VALUE     "\"${HPX_PARCELPORT_VERBS_DEVICE}\""
    NAMESPACE parcelport_verbs)

  hpx_add_config_define_namespace(
    DEFINE    HPX_PARCELPORT_VERBS_INTERFACE
    VALUE     "\"${HPX_PARCELPORT_VERBS_INTERFACE}\""
    NAMESPACE parcelport_verbs)

and then to generate a defines.hpp file as follows

  write_config_defines_file(
    TEMPLATE ""
    NAMESPACE "parcelport_verbs"
    FILENAME "${CMAKE_BINARY_DIR}/hpx/config/parcelport_verbs_defines.hpp")

when no template is specified, the function will generate a copyright
block, and include guard #ifndef...#endif section with the #defines between.
In the above example, the following would be generated from the variables

  #define HPX_PARCELPORT_VERBS_DEVICE "mlx5_0"
  #define HPX_PARCELPORT_VERBS_INTERFACE "ib0"

This feature allows a module (e.g. parcelport) to generate #defines without
writing to the global defines.hpp file and triggering full rebuilds on every change.

[hkaiser]

LGTM, nice idea!

[hkaiser]

This fixes #2397