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

diff --git a/auto/cc/clang b/auto/cc/clang
index 0cca344..baaf4ba 100644
--- a/auto/cc/clang
+++ b/auto/cc/clang
@@ -89,7 +89,7 @@
 CFLAGS="$CFLAGS -Wno-unused-parameter"
 
 # stop on warning
-#CFLAGS="$CFLAGS -Werror"
+CFLAGS="$CFLAGS -Werror"
 
 # debug
 CFLAGS="$CFLAGS -g"