[PATCH] Fix build if __GLIBC_PREREQ is not defined

[Jörg Krause]

Hi Thorsten,

On Fri, 2018-03-02 at 12:46 +0000, Thorsten Glaser wrote:
> Jörg Krause dixit:
> 
> > Fixes build error with musl:
> 
> musl doesn't define __GLIBC__ so the second check in...
> 
> > #if defined(_GNU_SOURCE) && defined(__GLIBC__) && !__GLIBC_PREREQ(2, 26)
> 
> ... suffices already.

No. Before evaluation, all macros are expanded. In case of musl, where
__GLIBC_PREREQ is not defined, the macro is expanded to 0 resulting in
the following invalid expression:

#if 1 && 0 && !0(2, 26)

> If you run into this, it must be a local problem,
> i.e. someone defined __GLIBC__ on a non-glibc system,
> since cpp short-circuits such an evaluation, and this
> is the documented way to use __GLIBC_PREREQ().

Jörg