When LibXC is compiling for Python, as many as possible threads are used

Many machines, especially scientistic, now have many threads (>=8). Probably, it was the reason to set -j8 in setup.py. However, in CI, the typical count of threads is 1-2. So, running of pip install libxc leads to overloading of the machine. Since this patch, the count of threads for compilation will be selected automatically according to available threads.