wrapped the CRT warning macro

I already have these in an outer-script

tools/assimp_cmd/Main.h

@@ -46,7 +46,9 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #ifndef AICMD_MAIN_INCLUDED
 #define AICMD_MAIN_INCLUDED
 
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
+#endif
 
 #include <stdio.h>
 #include <string.h>