configure: default --enable-valgrind to auto

... as the help text says.

Reported-by: Jasper St. Pierre <jstpierre@mecheye.net>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
diff --git a/configure.ac b/configure.ac
index 1456b90..001fd3d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -406,7 +406,7 @@
 AC_ARG_ENABLE(valgrind,
               [AS_HELP_STRING([--enable-valgrind],
                              [Build libdrm with  valgrind support (default: auto)])],
-                             [VALGRIND=$enableval], [VALGRIND=yes])
+                             [VALGRIND=$enableval], [VALGRIND=auto])
 PKG_CHECK_MODULES(VALGRIND, [valgrind], [have_valgrind=yes], [have_valgrind=no])
 AC_MSG_CHECKING([whether to enable Valgrind support])
 if test "x$VALGRIND" = xauto; then