Do not expand BOOST_{CXX14_}CONSTEXPR while preprocessing.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Fix forward declaration in conflict with constexpr.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Make fold as a single return stmt due to C++11 constexpr.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Use BOOST_CXX14_CONSTEXPR instead of BOOST_CONSTEXPR.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Merge pull request #23 from Flast/bugfix/cxx14-constexpr

Improve constexpr supports