Summary:
Recent changes added an include path in the float128 type that used the
internal `libc` path to find the macro. This doesn't work once it's
installed because we need to search from the root of the install dir.
This patch adds "include/" to the include path so that our inclusion
of installed headers always match the internal use.
Floating point properties are a combination of target OS, target
architecture and compiler support.
- Adding target OS detection,
- Moving floating point type detection to its own file.
This is in preparation of adding support for `_Float16` which requires
testing compiler **version** and target architecture.
