meson.build: Switch to C++14

C++14 is a minor release that doesn't introduce major new concepts or
paradigms compared to C++11, but brings two useful changes for us:

- std::make_unique allows dropping our custom implementation in utils.
- Functions returning constexpr are not assumed to be const anymore,
  which is needed to create a standard-conformant span implementation.

All the g++ and clang++ versions we support and test (g++-5 onwards and
clang++6 onwards) support C++14. However, due to a defect in the
original C++14 specification, solved in N4387 ([1]), compilation would
fail on g++-5 due to the use of std::map::emplace() with a non-copyable
value type. It turns out we can easily fix it by switching to the
explicit piecewise emplace() overload.

There is thus really nothing holding back the switch. Let's do it, and
update the coding style accordingly.

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4387

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

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