## `c_std` and `cpp_std` options now accepts a list of values

Projects that prefer GNU C, but can fallback to ISO C, can now set, for
example, `default_options: 'c_std=gnu11,c11'`, and it will use `gnu11` when
available, but fallback to `c11` otherwise. It is an error only if none of the
values are supported by the current compiler.

Likewise, a project that can take benefit of `c++17` but can still build with
`c++11` can set `default_options: 'cpp_std=c++17,c++11'`.

This allows us to deprecate `gnuXX` values from the MSVC compiler. That means
that `default_options: 'c_std=gnu11'` will now print a warning with MSVC
but fallback to `c11`. No warning is printed if at least one
of the values is valid, i.e. `default_options: 'c_std=gnu11,c11'`.

In the future that deprecation warning will become an hard error because
`c_std=gnu11` should mean GNU is required, for projects that cannot be
built with MSVC for example.