Fix that option '--enable-rtree' is implied by '--enable-all' as documented for the latter option. Thanks.

diff --git a/configure.ac b/configure.ac
index 516009341..446be2f27 100644
--- a/configure.ac
+++ b/configure.ac
@@ -669,7 +669,7 @@ fi
 # See whether we should enable RTREE
 AC_ARG_ENABLE(rtree, AS_HELP_STRING([--enable-rtree],
       [Enable the RTREE extension]))
-if test "${enable_rtree}" = "yes" ; then
+if test "${enable_rtree}" = "yes" || test "${enable_all}" = "yes"; then
   OPT_FEATURE_FLAGS="${OPT_FEATURE_FLAGS} -DSQLITE_ENABLE_RTREE"
 fi