Not all compiler support all C++20's components when using `-std=c++20`

[owent]

Describe your environment

OS: Linux
Compiler: GCC 11

Steps to reproduce
Compiling otel-cpp with -std=c++20

What is the expected behavior?
Building success.

What is the actual behavior?

/home/runner/work/cmake-toolset/cmake-toolset/test/third_party/packages/opentelemetry-cpp-v1.18.0/exporters/elasticsearch/src/es_log_recordable.cc:181:36: error: ‘format’ is not a member of ‘std’
  181 |   const std::string dateStr = std::format("{:%FT%T%Ez}", timePoint);
      |                                    ^~~~~~
gmake[2]: *** [exporters/elasticsearch/CMakeFiles/opentelemetry_exporter_elasticsearch_logs.dir/build.make:93: exporters/elasticsearch/CMakeFiles/opentelemetry_exporter_elasticsearch_logs.dir/src/es_log_recordable.cc.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:1593: exporters/elasticsearch/CMakeFiles/opentelemetry_exporter_elasticsearch_logs.dir/all] Error 2

Additional context
https://github.com/owent/cmake-toolset/actions/runs/12311600885/job/34362063562

There may be other errors

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
[cmake] Please set them or make sure they are set and tested correctly in the CMake files:
[cmake] GMOCK_LIB
[cmake]     linked by target "url_parser_test" in directory D:/workspace/git/github/opentelemetry/opentelemetry-cpp/ext/test/http
[cmake]     linked by target "curl_http_test" in directory D:/workspace/git/github/opentelemetry/opentelemetry-cpp/ext/test/http
[cmake] 
[cmake] CMake Generate step failed.  Build files cannot be regenerated correctly.

[lalitb]

I'm wondering if features from C++17 and later should be conditionally allowed in the code using preprocessor directives (as in this case), as addressing errors related to them could be challenging.

[owent]

I'm wondering if features from C++17 and later should be conditionally allowed in the code using preprocessor directives (as in this case), as addressing errors related to them could be challenging.

The library feature-test macros are only be defined after the headers are included. But we can not include it when it's not exists. The document says the macros will also be defined when <version> is inclued.
I will test it.