Build examples from CMake rather than executable

[mjcarroll]

🦟 Bug fix

Summary

Part of gazebosim/gz-cmake#301

Test by building gz-math as normal and then running the "EXAMPLES_Configure_TEST" and "EXAMPLES_Build_TEST" via ctest.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[mjcarroll]

Tested locally on Windows with:

colcon build --packages-up-to gz-math7 --merge-install
Summary: 3 packages finished [5 min 20s]

colcon test --packages-select gz-math7 --merge-install
Summary: 1 package finished [31.9s]

I also found a neat side effect, this lets us additionally turn on ninja on windows:

colcon build --packages-up-to gz-math7 --merge-install
Summary: 3 packages finished [1 min 1 s]

colcon test --packages-select gz-math7 --merge-install
Summary: 1 package finished [22.1s]

[mjcarroll]

@osrf-jenkins retest this please

[mjcarroll]

@osrf-jenkins retest this please

[ahcorde]

Changes LGTM, but CI is ugly, all python tests are failing

[codecov]

Codecov Report

Merging #482 (482ad67) into gz-math7 (cf5d7f3) will decrease coverage by 5.36%.
The diff coverage is 100.00%.

❗ Current head 482ad67 differs from pull request most recent head cedd4a4. Consider uploading reports for the commit cedd4a4 to get more accurate results

@@             Coverage Diff              @@
##           gz-math7     #482      +/-   ##
============================================
- Coverage     99.70%   94.34%   -5.36%     
============================================
  Files            77      144      +67     
  Lines          7049     9660    +2611     
============================================
+ Hits           7028     9114    +2086     
- Misses           21      546     +525     

Impacted Files	Coverage Δ
include/gz/math/Helpers.hh	100.00% <100.00%> (ø)
include/gz/math/Polynomial3.hh	100.00% <100.00%> (ø)

... and 82 files with indirect coverage changes