Fix HPX-APEX cmake integration

[srinivasyadav18]

Existing apex cmake integration does not work for external Apex installation.

This fix seperates the git fetch installation using HPX_WITH_FETCH_APEX and external installation using Apex_ROOT, cmake variables respectively.

[hkaiser]

LGTM, thanks!

[hkaiser]

This makes some of the builders fail:

• https://github.com/STEllAR-GROUP/hpx/actions/runs/6065900150/job/16455889171?pr=6342
• https://app.circleci.com/pipelines/github/STEllAR-GROUP/hpx/15729/workflows/5b3db204-05da-4776-b9c2-814f42beb9c7/jobs/361491

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
-0.06%	∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (2f81937)	188353	160357	85.14%
Head commit (7a505ab)	188353 (+0)	160234 (-123)	85.07% (-0.06%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#6342)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[srinivasyadav18]

Details

This makes some of the builders fail:

• https://github.com/STEllAR-GROUP/hpx/actions/runs/6065900150/job/16455889171?pr=6342
• https://app.circleci.com/pipelines/github/STEllAR-GROUP/hpx/15729/workflows/5b3db204-05da-4776-b9c2-814f42beb9c7/jobs/361491

@hkaiser I have fixed the issues. I think it is now ready to be merged. Please review it. Thank you!

[hkaiser]

@srinivasyadav18 the cmake formatting needs some touching up, still

[srinivasyadav18]

@srinivasyadav18 the cmake formatting needs some touching up, still

I have just rebased the branch. That fixed the issue. Thanks!

[hkaiser]

bors merge

[bors]

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