Remove use of Boost.Math, to avoid the dependency cycle

[pdimov]

This patch replaces the use of isnan, changesign, signbit, isinf from Boost.Math with <math.h>. I've tested it locally on Windows under msvc-8.0, msvc-10.0, msvc-11.0, msvc-12.0, msvc-14.0, msvc-14.1, clang-win, gcc (cygwin). Of these, it breaks msvc-8.0, 10.0, 11.0, because they don't have the functions in <math.h>, and introduces no new failures on the rest.

Experiments on Godbolt suggest that all Linux gcc versions starting from 4.1 (earliest on Godbolt) have these functions. E.g. https://godbolt.org/z/WTMGr9

Linux Clang, starting from 3.0 (earliest on Godbolt), also seems to work: https://godbolt.org/z/nr1Tvo

What hasn't been tested: Apple Clang, any version.

[pdimov]

Added emulation math functions for msvc-11.0 and below in a separate commit. With this additional change, msvc-8.0/10.0/11.0 are now unbroken (have no more failures than before.)

[pdimov]

It's not so easy, as it turns out.

[pdimov]

OK, now tested on Travis. The results of the current develop branch, as a baseline, are here: https://travis-ci.com/github/pdimov/lexical_cast/builds/203465518

And the results with this PR applied are here: https://travis-ci.com/github/pdimov/lexical_cast/builds/203465561

As far as I can see, it doesn't introduce any new errors. The macOS and clang-ubsan failures are preexistent. They should probably be cleaned up, but that's a separate exercise.