Use C++11 std::isnan and std::isfinite, and don't include an unneeded header on linux.

[leper]

This allows building against musl libc.

[castano]

I would prefer to not add dependencies on additional C++ headers. Is that necessary to handle any particular platform? Removing sysctl if not used seems like a good idea, though.

[leper]

You already pull in <cmath> via src/nvtt/squish/maths.h in a few places, also <cmath> is in some ways just a C++/namespace std wrapper around C's <math.h>. And C99 and C++11 standardize isnan/isfinite, but requiring C99 in a C++ project is usually not something you want to do, especially on Windows where C is only getting C99 additions due to C++11.

The code would actually remove the other isnan and isfinite checks if C++11 could be used as a minimum required standard for building the library. But I do not know what platforms you actually care to support or what the minimum requirements for this library are.

This change is required to build on Linux with musl libc (e.g. Alpine Linux, Void Linux). The change itself has been tested by CI for a bundled copy patched with this on multiple platforms all using C++11 (or later) including Windows, Linux+glibc, Linux+musl, FreeBSD, macOS (which does need #270 to build).