Use BOOST_INLINE_CONSTEXPR for inline const variables

[matbech]

This fixes the data duplication in translation units when compiling with C++17 or newer.
Fixes issue #20

[georgthegreat]

@pdimov it looks like this PR broke boost/graph:

$S/graph/src/read_graphviz_new.cpp failed with exit code 1 in /home/thegeorg/.ya/build/build_root/nj9s/000004
In file included from $Sgraph/src/read_graphviz_new.cpp:49:
In file included from $Sgraph/include/boost/graph/graphviz.hpp:36:
In file included from $Sxpressive/include/boost/xpressive/xpressive_static.hpp:24:
$Sxpressive/include/boost/xpressive/regex_primitives.hpp:243:71: error: constexpr variable 'epsilon' must be initialized by a constant expression
  243 | BOOST_INLINE_CONSTEXPR proto::terminal<detail::epsilon_matcher>::type epsilon = {{}};
      |                                                                       ^         ~~~~
$Sxpressive/include/boost/xpressive/regex_primitives.hpp:243:81: note: non-constexpr constructor 'expr' cannot be used in a constant expression
  243 | BOOST_INLINE_CONSTEXPR proto::terminal<detail::epsilon_matcher>::type epsilon = {{}};

[pdimov]

Broke Graph under what compiler with what options? Where does this build log come from?