[R-SIG-Mac] 'math.h' file not found

[Roger Koenker]

I have an annoying installation issue with a new mac mini:  for example I see:

> In file included from /usr/local/clang6/include/c++/v1/cmath:305:
> /usr/local/clang6/include/c++/v1/math.h:301:15: fatal error: 'math.h' file not found
> #include_next <math.h>
>               ^~~~~~~~
> 1 error generated.

even though:

> yzzy: ls -l /usr/local/clang6/include/c++/v1/math.h
> -rw-r--r--  1 root  wheel  48240 Mar  2  2018 /usr/local/clang6/include/c++/v1/math.h

this is with clang tools installed from Simon’s page and after trying various sorts of 
voodoo from googling like 

>xyzzy:  xcode-select --install
xcode-select: error: command line tools are already installed, use "Software Update" to install updates


R version 3.5.1 (2018-07-02) -- "Feather Spray"
Copyright (C) 2018 The R Foundation for Statistical Computing
Platform: x86_64-apple-darwin15.6.0 (64-bit)

Any hints appreciated, and apologies if this should have been in R-devel