Add old style find boost applying option to cmake.

[redboltz]

When cmake has -D MSGPACK_BOOST_USE_OLD_CMAKE=ON option, cmake use old style find boost.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.36%. Comparing base (e18398a) to head (12dd726).
⚠️ Report is 14 commits behind head on cpp_master.
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Additional details and impacted files

@@              Coverage Diff               @@
##           cpp_master    #1172      +/-   ##
==============================================
- Coverage       86.07%   85.36%   -0.71%     
==============================================
  Files              80       80              
  Lines            5392     5099     -293     
  Branches          503      507       +4     
==============================================
- Hits             4641     4353     -288     
+ Misses            751      746       -5     

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