Unbreak the build: use std::isfinite

[barracuda156]

Fixes: #17

[barracuda156]

CI failures unrelated to the PR.

[tdhock]

hi thanks for the PR! can you please clarify what build this would fix? I do not see the warning in that linked issue in any builds.

[barracuda156]

@tdhock Relevant for older macOS; analogous to isnan issue here, for example: AcademySoftwareFoundation/openexr#1448

AFAIK, this should not have any adverse affects for other systems; of course, if any such concerns exist, the fix should be conditional.

[barracuda156]

P. S. On macOS this is confirmed to build fine across every system from 10.6 through 13: https://ports.macports.org/port/R-binsegRcpp/details
PowerPC build confirmed locally (no buildbot for it at the moment).
We apply the fix unconditionally, so it works both with Clang and GCC: https://github.com/macports/macports-ports/blob/master/R/R-binsegRcpp/Portfile

[barracuda156]

@tdhock Thanks for merging!