Thread-safety tests

[Jamesits]

[Klevh]

Can be done easily :

• lock a mutex whenever you use a global variable, unlock it when you are done

[Jamesits]

@Klevh I guess this needs some threading library... I wanted it to be plain C implementation.

[brvtalcake]

You can also put all globals in a single C source file (not header), as static thread_local, and write basic functions to access them (it just works the same as encapsulation in C++, for example), so :

1. Coupling is (a bit) reduced
2. Each thread has its own exceptions