Propagate macro definitions from opentelemetry-cpp::api to all dependents

[ThomsonTan]

This is for follow-up for below comment

#3440 (comment)

[github-actions]

This issue was marked as stale due to lack of activity.

[dbarker]

The API CMake target has been linked publicly to the dll target and addresses this issue. Closing.

opentelemetry-cpp/ext/src/dll/CMakeLists.txt

Line 20 in 21801c2

PUBLIC opentelemetry_api