I am allowed to modify the Coin library itself?!

Yup. You are allowed to do that, for instance in order to fix bugs or adapt it to your special needs. You are of course not allowed to make a modified version of Coin and sell it as your own product. Note that if you have made modifications to the Coin library, you are not allowed to distribute the modified source code, you have to use source code diffs (patches). You are free to distribute a binary of the changed version without the need to disclose your source code, but then you can't name that binary "Coin". This is necessary to avoid conflicts and confusion.