[gcc:11] `is{nan,inf}` not declared in this scope.

[bartoszek]

Fail to build against gcc:11.1.0 ( env:{os:ArchLinux,gcc:11.1.0} ) Compiler complains about is{nan,isinf} being undefined and suggest replacing with std::&

Relevant files:

include/luxrays/core/color/color.h
include/luxrays/core/color/swcspectrum.h
include/luxrays/core/geometry/normal.h
include/luxrays/core/geometry/point.h
include/luxrays/core/geometry/uv.h
include/luxrays/core/geometry/vector.h
include/luxrays/utils/utils.h
include/slg/film/framebuffer.h