Project file for macOS requires FBX SDK

Project file config that we created for 1.0 RC1 expects FBX SDK to always be present on macOS. To fix this, the macOS .pro file part that includes the FBX lib needs to be made conditional. See task: https://codereview.qt-project.org/#/c/211615/2/src/Authoring/Studio/Qt3DStudio.pro comments for more details.