bug#32767: glibc shadows gcc's C++ headers

[Robin Templeton <robin@igalia.com>]

When compiling C++ programs, glibc's include directory takes precedence
over gcc's for standard C headers like math.h, but glibc's headers
aren't completely compatible with C++. For example, isnan from math.h is
supposed to be a function, but glibc defines it as a macro.

This program demonstrates the problem:

#include <math.h>
int main(void) { int isnan(0); return isnan; }

It works as expected when gcc's built-in C++ headers are used, but not
if glibc is installed:

% guix environment --pure --ad-hoc gcc -- g++ -E isnan.cpp | tail -n1 
int main (void) { int isnan (0); return isnan; }
% guix environment --pure --ad-hoc gcc glibc -- g++ -E isnan.cpp | tail -n1
int main (void) { int __builtin_isnan (0); return isnan; }

As a temporary workaround, I'm using the following package as a
replacement for glibc, to keep the glibc headers out of $CPATH. If it's
installed along with gcc, ld-wrapper and binutils, the test program
compiles without errors.

(use-modules (guix) (gnu))
(use-package-modules base)
(package
  (inherit glibc)
  (name "my-glibc")
  (arguments
   (substitute-keyword-arguments (package-arguments glibc)
     ((#:phases phases)
      `(modify-phases ,phases
         (add-after 'install 'move-include
           (lambda _
             (rename-file (string-append %output "/include")
                          (string-append %output "/include-glibc"))
             #t)))))))