Don't test for C++11 features when the compiler doesn't support it
(or the user has explicitly disabled it).

In the former case, it's usually a foregone conclusion that the tests
will fail, in the latter we have seen some compilers (Clang 3.5.0)
that still support certain C++11 features like std::shared_ptr even
when the user has not passed the -std=c++11 flag.  To avoid confusion,
it's best to enforce C++03 code where possible for such compilers.

Note that you must call AM_CONDITIONAL outside of conditionals...
i.e. outside of the 'if enablecxx11==yes' check. Hence all the
re-indenting.
Ran bootstrap.
Merge pull request #374 from libMesh/disable_cxx11

Disable C++11 features when the user says --disable-cxx11