On systems where gnutls uses libgcrypt, I'm seeing the following
build failure

libvirt.c:314: error: variable 'virTLSThreadImpl' has initializer but incomplete type
libvirt.c:319: error: 'GCRY_THREAD_OPTION_PTHREAD' undeclared here (not in a function)
...

Fix by undefining WITH_GNUTLS_GCRYPT in config-post.h