Fixes for MinGW compatibility

Some versions of MinGW do not support enough of Win32 API to let us work
with SEH, so SEH is now MSVC only (+ configurable toggle).

Also made use of gmtime_s MSVC only (as oposed to Windows only).

Fixes #805

include/reporters/catch_reporter_junit.hpp

@@ -26,7 +26,7 @@ namespace Catch {
             std::time(&rawtime);
             const size_t timeStampSize = sizeof("2017-01-16T17:06:45Z");
 
-#ifdef CATCH_PLATFORM_WINDOWS
+#ifdef _MSC_VER
             std::tm timeInfo = {};
             gmtime_s(&timeInfo, &rawtime);
 #else
@@ -37,7 +37,7 @@ namespace Catch {
             char timeStamp[timeStampSize];
             const char * const fmt = "%Y-%m-%dT%H:%M:%SZ";
 
-#ifdef CATCH_PLATFORM_WINDOWS
+#ifdef _MSC_VER
             std::strftime(timeStamp, timeStampSize, fmt, &timeInfo);
 #else
             std::strftime(timeStamp, timeStampSize, fmt, timeInfo);