The previous definition was used in C++ mode, which causes problems
when using clang with libc++ (rather than libstdc++), due to bugs
therein.  So just avoid in C++ mode.

A second problem is that depending on include order and implicit
includes the previous definition did not guarantee that the current
hack was effective by the time isinf was used, fix that by forcing
math.h to be included.  This can cause clang using builds, or gcc
using ones with JIT enabled, to slow down noticably.

It's likely that we at some point want a better solution for the
performance problem, but while it's there it should better work.

Reported-By: Steven Winfield
Bug: #15270
Discussion: https://postgr.es/m/153116283147.1401.360416241833049560@wrigleys.postgresql.org
Author: Andres Freund
Backpatch: 11, like the previous commit.