Support for extra.txt file on thirdparty dir

The extra.txt file contains a list of directories (separated by ';')
containing a library.
It is useful if you have a library that can not be in the thirdparty dir
or if you want to share the same library among many projects.

Example extra.txt file:

    C:\Boost\;
    C:\Qt\4.8.2;
    C:\MyOtherProject\thirdparty\glew