Compilation fails on Apple iOS v11.3 target

I tried compiling a project to iOS, using `iphoneOS 11.3 sdk`, build failed with the following error:

```
Error: /usr/local/lib/haxe/lib/hxcpp/3,4,188/src/hx/libs/std/Sys.cpp:271:17: error: call to unavailable function 'system': not available on iOS
int result = system(cmd.__s);
^~~~~~
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.3.sdk/usr/include/stdlib.h:195:6: note: candidate function has been explicitly made unavailable
int system(const char *) __DARWIN_ALIAS_C(system);
```