build: Fix bug with enabling use of system-installed boost libraries

When the flag is specified, the script did not properly set
``want_system_boost`` variable.

Also, ndnboost/ doesn't have enable_shared_from_this header...

refs #1126 (http://redmine.named-data.net/issues/1126)

Change-Id: I6c68a298f5343762e3631f5731aa3252dcf51f6b
diff --git a/configure.ac b/configure.ac
index 0e05e66..e938039 100644
--- a/configure.ac
+++ b/configure.ac
@@ -63,6 +63,7 @@
       [do not attempt to use system-installed boost libraries and use the bundled version (ndnboost) when necessary.]
   ),
   [
+  want_system_boost="yes"
   if test "$withval" = "no"; then
       want_system_boost="no"
   fi