do not assume x3dgc to be the highest level project.

Without doing this change cmake would always look for the x3dgc folders to be at the root of the project I am using.
In my project x3dgc is a dependency in another folder and I add the dependency doing this `add_subdirectory(dependencies/x3dgc/src)`.

detail: I made a minimal change, but maybe you would like to use upper case for x3dgc project name so that when concatenated with _SOURCE_DIR it looks better...