[OpenMP] Silence build warnings when built with MinGW

[mstorsjo]

Summary

I've tried to upstream these changes first in
https://github.com/intel/ittapi/pull/25, but there hasn't been
any response whatsoever.

[AndreyChurbanov]

LGTM

openmp/runtime/src/thirdparty/ittnotify/disable_warnings.h

 
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "ittnotify_config.h"
 
 #if ITT_PLATFORM == ITT_PLATFORM_WIN
 
+#if defined _MSC_VER
+
 #pragma warning(disable : 593) /* parameter "XXXX" was set but never used */
 #pragma warning(disable : 344) /* typedef name has already been declared (with \
                                   same type) */
 #pragma warning(disable : 174) /* expression has no effect */
 #pragma warning(disable : 4127) /* conditional expression is constant */
 #pragma warning(                                                               \
     disable : 4306) /* conversion from '?' to '?' of greater size */
 
+#endif /* _MSC_VER */
+
 #endif /* ITT_PLATFORM==ITT_PLATFORM_WIN */
 
 #if defined __INTEL_COMPILER
 
 #pragma warning(disable : 869) /* parameter "XXXXX" was never referenced */
 #pragma warning(disable : 1418) /* external function definition with no prior  \
                                    declaration  */
 #pragma warning(                                                               \
     disable : 1419) /* external declaration in primary source file */
 
 #endif /* __INTEL_COMPILER */