Require C++11 if needed for gravl
Require C++11 if needed for gravl

Change how vistk does find_gravl to afterwards test if
VISTK_ENABLE_CXX11 is ON, if gravl was built in C++11 mode.

mathstuf/sprokit,Kitware/sprokit,Kitware/sprokit,mathstuf/sprokit,Kitware/sprokit,Kitware/sprokit,linus-sherrill/sprokit,linus-sherrill/sprokit,mathstuf/sprokit,linus-sherrill/sprokit,mathstuf/sprokit,linus-sherrill/sprokit