Symbol management in header files

[Florian Weimer]

Historically, -D_GNU_SOURCE turns out all optional features in header 
files.  All in all, this is a good thing because it makes simple for 
programmers, and something like this is needed for using glibc features 
in header-only libraries.  No one wants repeats of strerror_r, basename, 
or snprintf.

However, TS 18661-1 added “iszero” to <math.h>, and this causes some 
build failures.  Apparently, the main problem here is that it is a 
macro, which means it is not subject to scoping (which helps C++ in 
particular to cope with definitions of nextup/nextdown/fsub/canonincalize).

Is there anything we can do about this?  Do we need some guidance for 
dealing with standards which define macros with generic names?

(Let's keep the discussion here focused on source-code compatibility; I 
will start another thread about symbol management at the ELF level.)

Thanks,
Florian