Compile STIR with C++11 but force boost::shared_ptr

Current STIR master will use std::shared_ptr by default when using
C++-11. That breaks SIRF (see Issue #60). As work-around, we force
usage of boost::shared_ptr.

It might not be necessary to compile STIR with C++11, but
it now has some ugly work-arounds related to unique_ptr if that isn't
available which can break SIRF.

SuperBuild/External_STIR.cmake

@@ -51,6 +51,8 @@ if(NOT ( DEFINED "USE_SYSTEM_${externalProjName}" AND "${USE_SYSTEM_${externalPr
         -DBOOST_ROOT=${BOOST_ROOT}
         -DCMAKE_INSTALL_PREFIX=${STIR_Install_Dir}
         -DGRAPHICS=None
+        -DCMAKE_CXX_STANDARD=11
+        -DSTIR_USE_BOOST_SHARED_PTR=On
     INSTALL_DIR ${STIR_Install_Dir}
     DEPENDS
         ${${proj}_DEPENDENCIES}