With which files does Eclipse start to compile and how to change it

Question

I'm importing a different project into mine - that has not been properly coded when it comes to include files. When compiling, it generates random errors that some functions/variables etc. are not defined.

I believe this is because of a wrong compile/include order, because the missing declarations are in files that are actually part of the project.

So how does Eclipse (CDT) make it ? Is it aphabetical order? How can I change it, do I really have to write my own makefile, or is there a way to force the compiler to start with a specific file ?

Kind Regards