Fix macOS linker warnings

[fasmat]

Motivation

This should fix linker warnings we keep seeing on macOS.

Merge after https://github.com/spacemeshos/post/pull/297

Description

Update post library and remove duplicate LDFLAGS definition in Makefile.

Test Plan

• existing tests pass.

TODO

• [x] Explain motivation or link existing issue(s)
• [x] Test changes and document test plan
• [x] Update documentation as needed
• [x] Update changelog as needed

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.7%. Comparing base (517c305) to head (dc10746). Report is 11 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6095     +/-   ##
=========================================
- Coverage     79.7%   79.7%   -0.1%     
=========================================
  Files          328     328             
  Lines        42904   42904             
=========================================
- Hits         34224   34208     -16     
- Misses        6752    6764     +12     
- Partials      1928    1932      +4     

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[fasmat]

Since the fix only works after we update our CI runners, I'll set this to draft for now.

[fasmat]

Abandoning for now. The warning are annoying but getting rid of them seems not to be worth the effort.