Add a note on the ACX_PTHREAD macro license.

COPYRIGHT

@@ -53,3 +53,6 @@ HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 SUCH DAMAGE.
+
+Also the ACX_PTHREAD macro in the file acinclude.m4 is distributed under
+the GPLWithACException license detailed on the Autoconf Macro Archive site.