How to add an external library in a project?
For those who may have the same problem regarding symbols, I have understood my mistake:
To define them properly, define them in Project > Properties > C/C++ General > Paths & Symbols
Configuration : | All configurations | (this is where I wasn't doing it correctly)
If you have to declare a symbol for a #if / #endif, add a value (1 for expl) to your symbol. Else, it won't work.
If you have to declare a symbol for a #ifdef, you don't need to add a value, as soon as you define it in the properties.
Hope it helps!