Draft: Add ALLOW_OPTIONAL_DEPENDENCIES to allow building without all dependencies

I am currently cross-compiling for a minimal CheriBSD system (a fork of FreeBSD) and I have not yet ported all depdencies. This commit adds a new option ALLOW_OPTIONAL_DEPENDENCIES that can be set to continue with the build and disable some features instead of failing the CMake checks. This currently affects the dependencies that are also optional for ANDROID.