Fix unknown pragma warnings on mingw
Client: cpp
Patch: Kevin Wojniak

This closes #1917

These pragmas are only valid for MSVC. Fixes warnings found in the logs:

> In file included from C:\projects\thrift\lib\cpp\test\processor\ProcessorTest.cpp:40:
731C:\projects\thrift\lib\cpp\test\processor\Handlers.h:143: warning: ignoring #pragma warning  [-Wunknown-pragmas]
732  143 |   #pragma warning( push )
733      |
734C:\projects\thrift\lib\cpp\test\processor\Handlers.h:144: warning: ignoring #pragma warning  [-Wunknown-pragmas]
735  144 |   #pragma warning (disable : 4250 ) //inheriting methods via dominance
736      |
737C:\projects\thrift\lib\cpp\test\processor\Handlers.h:172: warning: ignoring #pragma warning  [-Wunknown-pragmas]
738  172 |   #pragma warning( pop )
739      |
diff --git a/lib/cpp/test/processor/Handlers.h b/lib/cpp/test/processor/Handlers.h
index 05d19ed..d72a23c 100644
--- a/lib/cpp/test/processor/Handlers.h
+++ b/lib/cpp/test/processor/Handlers.h
@@ -139,7 +139,7 @@
   std::shared_ptr<EventLog> log_;
 };
 
-#ifdef _WIN32
+#ifdef _MSC_VER
   #pragma warning( push )
   #pragma warning (disable : 4250 ) //inheriting methods via dominance
 #endif
@@ -168,7 +168,7 @@
   int32_t value_;
 };
 
-#ifdef _WIN32
+#ifdef _MSC_VER
   #pragma warning( pop )
 #endif