Work around Python.h name-space pollution.
Work around Python.h name-space pollution.

* python/libvir.c (HAVE_PTHREAD_H): #undef.
Without this, we'd get a redefinition warning.

cardoe/libvirt-python,cardoe/libvirt-python,cardoe/libvirt-python,libvirt/libvirt-python,libvirt/libvirt-python,libvirt/libvirt-python