The build supports compiling with WITH_OTLP_GRPC_SSL_MTLS_PREVIEW, per api/CMakeLists:

if(WITH_OTLP_GRPC_SSL_MTLS_PREVIEW)
  target_compile_definitions(opentelemetry_api
                             INTERFACE ENABLE_OTLP_GRPC_SSL_MTLS_PREVIEW)
endif()

The problem is that this option, WITH_OTLP_GRPC_SSL_MTLS_PREVIEW, is not defined in the top level CMakeLists.txt.

When using ccmake, the UI does not expose this choice.