Use Boost namespaced targets in CMakeLists

[ashtum]

Without Boost namespace, the superproject cannot resolve dependencies from subdirectory CMakeLists:
boostorg/cmake#86

[cppalliance-bot]

An automated preview of the documentation is available at https://169.http-proto.prtest.cppalliance.org/index.html

[cppalliance-bot]

GCOVR code coverage report https://169.http-proto.prtest.cppalliance.org/gcovr/index.html
LCOV code coverage report https://169.http-proto.prtest.cppalliance.org/genhtml/index.html
Coverage Diff https://169.http-proto.prtest.cppalliance.org/gcovr/coverage_diff.txt

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.53%. Comparing base (63b0149) to head (b028fe2).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #169   +/-   ##
========================================
  Coverage    92.53%   92.53%           
========================================
  Files           81       81           
  Lines         5064     5064           
========================================
  Hits          4686     4686           
  Misses         378      378           

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[cppalliance-bot]

An automated preview of the documentation is available at https://169.http-proto.prtest.cppalliance.org/index.html

[cppalliance-bot]

GCOVR code coverage report https://169.http-proto.prtest.cppalliance.org/gcovr/index.html
LCOV code coverage report https://169.http-proto.prtest.cppalliance.org/genhtml/index.html
Coverage Diff https://169.http-proto.prtest.cppalliance.org/gcovr/coverage_diff.txt