Thanks to C++14, we can declare and fill variables inside `constexpr`
functions.  This means me can stop make assumptions on the `struct`
layouts without losing `constexpr`.

Closes #393

This fixes a build issue on Haiku as it does have sin_len.
Tested on Linux as well.

For some reason AC_CHECK_MEMBER doesn't generate the proper define
in config.h.in, so I used AC_CHECK_MEMBERS.