Fix include guard mismatch.

Signed-off-by: Kohei Takahashi <flast@flast.jp>
Merge pull request #21 from Flast/bugfix/include-guard-mismatch

Fix include guard mismatch.
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>
XXX: wip...

Signed-off-by: Kohei Takahashi <flast@flast.jp>