Configure: enabled -Werror for clang.

Modern clang versions seem to no longer produce warnings for
system headers on Linux (at least clang 3.3 works), hence the
change.  For older versions --with-cc-opt="-Wno-error" can be
used as a workaround.

auto/cc/clang

@@ -89,7 +89,7 @@ CFLAGS="$CFLAGS -Wconditional-uninitialized"
 CFLAGS="$CFLAGS -Wno-unused-parameter"
 
 # stop on warning
-#CFLAGS="$CFLAGS -Werror"
+CFLAGS="$CFLAGS -Werror"
 
 # debug
 CFLAGS="$CFLAGS -g"