Delegate the implementation of QSMath to <tgmath.h>

Thanks for [QSKit](https://github.com/quartermaster/QSKit)! I'm opening this pull request mostly as a question to see if you have any general feedback on `<tgmath.h>`.

[`<tgmath.h>`][1] provides type-generic macros for the functions found in `<math.h>` and `<complex.h>`, so out of the box it handles the branching logic of `QSMath`. If relying on a QS-based interface (e.g., `QSCeil`/`QSAbs`/`QSFloor`) isn't required, clients could directly import `<tgmath.h>` and use its macros.

All [`QSMathTests`][qmt] pass.

[1]: http://pubs.opengroup.org/onlinepubs/009696699/basedefs/tgmath.h.html
[qmt]: https://github.com/quartermaster/QSKit/blob/7a43cb9eb499035f1c3d9bec70e0b70171e62aa9/QSKitTests/QSMathTests.m