Bump CMake files to at least c++11.

Removed all configurations that explicitly test or set the c++ standard
flags. The only place the standard is now configured is at the top of
the main `CMakeLists.txt` file, which can easily be updated (e.g. if
we decide to move to c++14+). This can also be set via command-line using
```
> cmake -DCMAKE_CXX_STANDARD 14
```

Kept the `EIGEN_TEST_CXX11` flag for now - that still controls whether to
build/run the `cxx11_*` tests.  We will likely end up renaming these
tests and removing the `CXX11` subfolder.

unsupported/doc/examples/SYCL/CMakeLists.txt

@@ -20,7 +20,6 @@ else(EIGEN_SYCL_TRISYCL)
     set(CMAKE_CXX_COMPILER ${ComputeCpp_DEVICE_COMPILER_EXECUTABLE})
     string(REPLACE "-Wlogical-op" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
     string(REPLACE "-Wno-psabi" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
-    string(REPLACE "-ansi" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
   endif()
   list(APPEND COMPUTECPP_USER_FLAGS
       -DEIGEN_NO_ASSERTION_CHECKING=1