Fixing APEX dependencies for HPX modules

[sithhell]

This fixes STEllAR-GROUP/hpx#3805

[msimberg]

Note: I'm planning to change the target name to hpx_pp for consistency (we have hpx_init, hpx_wrap, hpx_<component_name>, etc.) still before the release. You might want to wait until that's in master.

[msimberg]

If we go ahead and merge STEllAR-GROUP/hpx#3811 we'll just need to change the name and remove the global include directory. @sithhell I opened a PR against this branch to change it.

@khuck going forward it's not very sustainable that you have to check before a release that everything is ok. We should really have CI for HPX with APEX enabled to catch these situations earlier. Last time I checked some tests in HPX timed out with APEX enabled but I never had time to look into them. Would you have time to help us get those sorted out sometime after the next release?

[khuck]

@msimberg absolutely - if you could set up an HPX+APEX CI that would be great. I am curious about any tests timing out, but it's been a long time since APEX was integrated into HPX so whatever issues there might have been are likely gone now.