shield windows defines, in case they are already defined

httplib.h

@@ -9,8 +9,12 @@
 #define _CPPHTTPLIB_HTTPLIB_H_
 
 #ifdef _MSC_VER
+#ifndef _CRT_SECURE_NO_WARNINGS
 #define _CRT_SECURE_NO_WARNINGS
+#endif //_CRT_SECURE_NO_WARNINGS
+#ifndef _CRT_NONSTDC_NO_DEPRECATE
 #define _CRT_NONSTDC_NO_DEPRECATE
+#endif //_CRT_NONSTDC_NO_DEPRECATE
 
 #ifndef SO_SYNCHRONOUS_NONALERT
 #define SO_SYNCHRONOUS_NONALERT 0x20