Update how to compile with -Werror

--enable-compile-warnings=error has been renamed to --enable-werror so
update the HACKING and the hacking.html to reflect that.

HACKING

@@ -73,9 +73,10 @@ things).
 and don't care much about released versions.
 
 (5) Run the automated tests on your code before submitting any changes. In
-particular, configure with compile warnings set to -Werror:
+particular, configure with compile warnings set to -Werror. This is done
+automatically for a git checkout; from a tarball, use:
 
-  ./configure --enable-compile-warnings=error
+  ./configure --enable-werror
 
 and run the tests:
 

docs/hacking.html.in

@@ -74,9 +74,11 @@
       <li>Make sure your patches apply against libvirt GIT.  Developers
         only follow GIT and don't care much about released versions.</li>
       <li><p>Run the automated tests on your code before submitting any changes.
-          In particular, configure with compile warnings set to -Werror:</p>
+          In particular, configure with compile warnings set to
+          -Werror.  This is done automatically for a git checkout; from a
+          tarball, use:</p>
 <pre>
-  ./configure --enable-compile-warnings=error
+  ./configure --enable-werror
 </pre>
         <p>
           and run the tests: