Suppress -Wexit-time-destructors in Catch code

This is another warning that follows test macros, making it painful to
suppress without leaking outside. Luckily clang's `_Pragma`
implementation works.

Should fix #308

include/internal/catch_test_case_tracker.hpp

@@ -17,6 +17,8 @@
 #include <vector>
 #include <stdexcept>
 
+CATCH_INTERNAL_SUPPRESS_ETD_WARNINGS
+
 namespace Catch {
 namespace TestCaseTracking {
 
@@ -364,4 +366,6 @@ using TestCaseTracking::IndexTracker;
 
 } // namespace Catch
 
+CATCH_INTERNAL_UNSUPPRESS_ETD_WARNINGS
+
 #endif // TWOBLUECUBES_CATCH_TEST_CASE_TRACKER_HPP_INCLUDED