[BUILD] CMake calls find_package(CURL) in multiple places

[marcalff]

Per the CMake makefiles:

./examples/http/CMakeLists.txt:find_package(CURL)
./exporters/otlp/CMakeLists.txt:  find_package(CURL REQUIRED)
./exporters/zipkin/CMakeLists.txt:find_package(CURL REQUIRED)
./ext/src/http/client/curl/CMakeLists.txt:find_package(CURL)
./ext/test/http/CMakeLists.txt:find_package(CURL)
./ext/test/w3c_tracecontext_test/CMakeLists.txt:find_package(CURL)
./CMakeLists.txt:    find_package(CURL)

CMake should not call find_package(CURL) so many times from nested directories.

Beside, the current logic is questionable:

find_package(CURL)
if(NOT CURL_FOUND)
  message(WARNING "Skipping example_w3c_tracecontext_test: CURL not found")
else()

If CURL is not found and BUILD_W3CTRACECONTEXT_TEST is set, the build should fail, not pass and do nothing.

Instead:

• Evaluate once IF CURL is needed, similar to USE_NLOHMANN_JSON
• Call find_package(CURL) once from the top level CMakeLists.txt
• Assume CURL_FOUND and related variables are already evaluated, in sub directories.

Implementing this cleanup will help to add the CURL dependency from different sources (git submodule, external project, alternate location).

[lalitb]

Thanks, agree CURL checks can be much simplified. Later, once #1145 and #1146 are implemented, with native HTTP client libraries used for Windows and Mac, CURL can be made mandatory only for POSIX.