Fix Sylvan Cannot be Compiled as a Submodule

[SSoelvsten]

Note that CMAKE_SOURCE_DIR and PROJECT_SOURCE_DIR are two different variables. The former is the root of the entire tree (possibly a dependent project) whereas the latter is the latest call to project() (which is the Sylvan root folder in this case).

• Does it now work again as a submodule? Yes, I have tested this works as intended on my benchmarking repository. With this fix, everything compiles and runs with the latest version of Sylvan.
• Does it break installation? No, sudo make install indeed copies the desired files to /usr/local/... as intended.

[jacopol]

Could this be related to / would this also fix #47 ?

[SSoelvsten]

This would not fix #47, no. I do not exactly know why ccmake did not change the value correctly, but you can alternatively provide the prefix at the terminal with cmake.

[trolando]

I'll pull it in, but I recall that there was a better way to detect if a project is the root project or a subproject.

[SSoelvsten]

Thanks!

You can use the PROJECT_IS_TOP_LEVEL Cmake variable to check if something is the root project, if that is the you are thinking of. But, I'm not sure I yet see how it can improve code quality.