Fix compilation with gcrypt < 1.4.2

Commit 33a198c1f6 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.

src/libvirt.c

@@ -291,7 +291,12 @@ static int virTLSMutexUnlock(void **priv)
 }
 
 static struct gcry_thread_cbs virTLSThreadImpl = {
+    /* GCRY_THREAD_OPTION_VERSION was added in gcrypt 1.4.2 */
+#ifdef GCRY_THREAD_OPTION_VERSION
     (GCRY_THREAD_OPTION_PTHREAD | (GCRY_THREAD_OPTION_VERSION << 8)),
+#else
+    GCRY_THREAD_OPTION_PTHREAD,
+#endif
     NULL,
     virTLSMutexInit,
     virTLSMutexDestroy,