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/.travis.yml b/.travis.yml
index ff9a328..a36b2ca 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,7 @@
   - travis_retry sudo apt-get install -qq libsqlite3-dev
 script:
   - autoreconf -f --install
-  - ./configure --without-c++11 --with-system-boost --with-tests
+  - ./configure --with-tests
   - make
   - sudo make install
   - sudo ldconfig
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
diff --git a/include/ndn-cpp-dev/common.hpp b/include/ndn-cpp-dev/common.hpp
index a651a40..5efd3a0 100644
--- a/include/ndn-cpp-dev/common.hpp
+++ b/include/ndn-cpp-dev/common.hpp
@@ -64,7 +64,6 @@
 (unset LANG; find . -type f -exec sed -i '' 's/ mpl_/ ndnboost_mpl_/g' {} +)
  */
 #include <ndnboost/shared_ptr.hpp>
-#include <ndnboost/enable_shared_from_this.hpp>
 #include <ndnboost/make_shared.hpp>
 namespace ndn { namespace ptr_lib = ndnboost; }