[BUILD] Error out when building DLL without MSVC (#3438)

Author: ThomsonTan

CHANGELOG.md

@@ -15,6 +15,9 @@ Increment the:
 
 ## [Unreleased]
 
+* [BUILD] Error out when building DLL without MSVC
+  [#3438](https://github.com/open-telemetry/opentelemetry-cpp/pull/3438)
+
 * [Metrics SDK] Use nostd::function_ref in AttributesHashMap
   [#3393](https://github.com/open-telemetry/opentelemetry-cpp/pull/3393)
 

ext/src/CMakeLists.txt

@@ -5,6 +5,6 @@ if(WITH_HTTP_CLIENT_CURL)
   add_subdirectory(http/client/curl)
 endif()
 
-if(MSVC AND DEFINED OPENTELEMETRY_BUILD_DLL)
+if(DEFINED OPENTELEMETRY_BUILD_DLL)
   add_subdirectory(dll)
 endif()

ext/src/dll/CMakeLists.txt

@@ -1,6 +1,12 @@
 # Copyright The OpenTelemetry Authors
 # SPDX-License-Identifier: Apache-2.0
 
+if(NOT MSVC)
+  message(
+    FATAL_ERROR
+      "The OpenTelemetry C++ SDK for Windows DLL is only supported on MSVC.")
+endif()
+
 set(OPENTELEMETRY_EXPORT_DEF
     "${CMAKE_CURRENT_BINARY_DIR}/opentelemetry_cpp.def")