Force include catch_platform.h before any if-defs

Closes #994
diff --git a/include/catch.hpp b/include/catch.hpp
index db1f366..c109926 100644
--- a/include/catch.hpp
+++ b/include/catch.hpp
@@ -22,6 +22,8 @@
 #  define CATCH_CONFIG_EXTERNAL_INTERFACES
 #endif
 
+#include "internal/catch_platform.h"
+
 #ifdef CATCH_IMPL
 #  ifndef CLARA_CONFIG_MAIN
 #    define CLARA_CONFIG_MAIN_NOT_DEFINED