Fix HPX_WITH_FETCH_BOOST

[Pansysk75]

Fixes following issues when HPX_WITH_FETCH_BOOST=ON:

• Boost being re-fetched on projects that would depend on HPX.
• Boost not compiling necessary libraries when a non header-only instance of Boost was needed

Now when HPX_WITH_FETCH_BOOST=ON Boost will be installed alongside HPX.

To fix:

• Duplicated code for figuring out which Boost libraries are needed
• Make hpx_boost_root-install.cmake contain relative path instead of absolute(?)

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.10%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d08354d)	222123	188895	85.04%
Head commit (0e1fe2d)	191114 (-31009)	162717 (-26178)	85.14% (+0.10%)

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 (#6511)	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]

/retest lsu

[srinivasyadav18]

/retest

[hkaiser]

@Pansysk75 could you rebase this onto master to resolve the conflicts, please?

[Pansysk75]

@Pansysk75 could you rebase this onto master to resolve the conflicts, please?

This PR should be discarded in favor of #6510.