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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