Compiling C++ library for an iOS project

I am new using C++ libraries, so it would be nice if you could answer with a bit more details so that I can understand it too.

My goal is to add an external C++ library, namely giac/xcas, into my iOS project.

I looked for some information and I came up to his point:

• create a static/dynamic library

• add this library to the project

• write an Objective C++ - Wrapper

So following the instructions on giac/xcas homepage I am able to build a .a-library for the x86_64 architecture. When adding to my project, Xcode gives obviously the message:

Building for iOS but the linked library 'libgiac.a' was built for macOS.

Now, I found out that I have to compile the c++ library for another architecture using flags.
And now here is the problem. I do not know how to achieve this. I think I shall go to the specific folder and configure via

Code Block 
./configure something here 

It would be nice if you can help me. Thanks in advance!