CMakeLists: Use PROJECT_SOURCE_DIR instead of CMAKE_SOURCE_DIR

This is more localized to what we want to enforce directory-wise with
the project. CMAKE_SOURCE_DIR indicates the root of the source tree, but
this would cause the wrong behavior if someone included yuzu as part of
a larger buildsystem (for whatever reason). Instead, we want to use the
directory where the "project(yuzu)" command was declared as the root
path reference.