Merge branch 'feature/cmake-rewrite-c++-standard-detection' into 'master'

[CMake] Rewrite C++ standard detection

This is a complete rewrite of the existing C++ standard version test
that tries the various `-std=c++xy` flags and tries to figure out which
version of the standard to enable.

The existing version has a few problems:

- it is not really extensible to newer standards, those will just lead
to a deeper and deeper nesting of `if` statements.

- More importantly, there is no way to say: I want C++11, but not C++14
  (or, in the future, I want C++14, but not C++17). Being able to do so
  is important for testing and compatibility reason.

- There is no easy way for downstream modules to require a newer version
  of the standard than the core modules.

## Test logic

The new version of the test separates the logic from the data (versions,
flag names, compiler test sources) and just iterates over lists of that
data, in descending order to make sure the newest available standard
gets picked. The numerical value of that standard is stored in the
variable `CXX_MAX_SUPPORTED_STANDARD` and the test stops. If the test
fails to find a working combination, it issues a warning to the user and
only records support for C++03.

The test can be influenced by two CMake variables:

- `DISABLE_CXX_VERSION_CHECK` already existed in the old version of the
  test. It completely disables all testing and requires the user to
  manually set `CXX_MAX_SUPPORTED_STANDARD` to the correct value.
  Moreover, any required compiler command line switches have to be
  manually added to `CMAKE_CXX_FLAGS`.

- `CXX_MAX_STANDARD` defines the maximum version of the standard that
  the build system will try to enable. With this new switch, it becomes
  possible to limit the compiler to an older version of the standard
  than what it could theoretically support. For now, this defaults to
  C++14.

## Requirements check with a new CMake function

In order to allow module authors to easily state their minimum version
requirements, there is a new CMake function
`dune_require_cxx_standard()` that any Dune module can call to require
support for at least a given C++ standard:

```
dune_require_cxx_standard(MODULE "dune-functions" VERSION 14)
```

If the compiler doesn't meet the requirements, the function will report
the required and the actually available standard versions and abort with
a fatal error.

Moreover, it knows about CXX_MAX_STANDARD and will tell the user if the
value of that variable is below the requirements. This avoids desperate
users that have a shiny beta of GCC 6 with C++17 support wondering why
their own module using shiny C++17 concepts stubbornly fails to build...

This fixes #16, and a backport to 2.4 will also be a fix for #15.

See merge request !46