Wrap MSVC-only warning pragmas in a macro.

There's a __pragma expression which allows this. Android builds us Windows with
MinGW for some reason, so we actually do have to tolerate non-MSVC-compatible
Windows compilers. (Clang for Windows is much more sensible than MinGW and
intentionally mimicks MSVC.)

MinGW doesn't understand MSVC's pragmas and warns a lot. #pragma warning is
safe to suppress, so wrap those to shush them. This also lets us do away with a
few ifdefs.

Change-Id: I1f5a8bec4940d4b2d947c4c1cc9341bc15ec4972
Reviewed-on: https://boringssl-review.googlesource.com/8236
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/test/file_test.h b/crypto/test/file_test.h
index e90cc86..a859127 100644
--- a/crypto/test/file_test.h
+++ b/crypto/test/file_test.h
@@ -20,19 +20,15 @@
 #include <stdint.h>
 #include <stdio.h>
 
-#if defined(_MSC_VER)
-#pragma warning(push)
-#pragma warning(disable: 4702)
-#endif
+OPENSSL_MSVC_PRAGMA(warning(push))
+OPENSSL_MSVC_PRAGMA(warning(disable: 4702))
 
 #include <string>
 #include <map>
 #include <set>
 #include <vector>
 
-#if defined(_MSC_VER)
-#pragma warning(pop)
-#endif
+OPENSSL_MSVC_PRAGMA(warning(pop))
 
 // File-based test framework.
 //