Commit 33a198c1 increased the gcrypt
version requirement to 1.4.2 because the GCRY_THREAD_OPTION_VERSION
define was added in this version.

The configure script doesn't check for the gcrypt version. To support
gcrypt versions < 1.4.2 change the virTLSThreadImpl initialization
to use GCRY_THREAD_OPTION_VERSION only if it's defined.