devel/deforaos-libsystem is not jobs safe

Here the log shows a makefile race where it tries to create the same
symlink twice:

• libSystem.so.0 ---

ln -f -s -- libSystem.so.0.0 libSystem.so.0

• libSystem.so ---

ln -f -s -- libSystem.so.0.0 libSystem.so.0

• libSystem.so.0 ---

ln -f -s -- libSystem.so.0.0 libSystem.so

• libSystem.so ---

ln: libSystem.so.0: File exists