[PATCH] Fix build if __GLIBC_PREREQ is not defined

[Guillem Jover]

Hi!

On Wed, 2018-03-07 at 21:48:39 +0100, Jörg Krause wrote:
> On Wed, 2018-03-07 at 21:13 +0100, Jörg Krause wrote:
> > On Fri, 2018-03-02 at 08:03 +0100, Jörg Krause wrote:
> > > __GLIBC_PREREQ is specific to glibc so it should be checked if it is
> > > defined or not.
> > > 
> > > Fixes build error with musl:
> > > '''
> > > ../include/bsd/stdlib.h:70:66: error: missing binary operator before token "("
> > >  #if defined(_GNU_SOURCE) && defined(__GLIBC__) && !__GLIBC_PREREQ(2, 26)

> > I've noticed that this patch allows building with musl, but produces
> > warnings because of implicit function declarations.
> > 
> > An improved version of the patch could be for example:
> > 
> > '''
> > #if defined(__GLIBC__) && defined(__GLIBC_PREREQ)
> > #if __GLIBC_PREREQ(2, 25)
> > #define HAVE_EXPLICIT_BZERO
> > #endif
> > #endif
> > 
> > __BEGIN_DECLS
> > [...]
> > 
> > #if defined(_GNU_SOURCE) && !defined(HAVE_EXPLICIT_BZERO)
> > void explicit_bzero(void *buf, size_t len);
> > #endif
> > __END_DECLS
> > '''AC_CHECK_FUNCS
> > 
> > Would do you think?

When I saw your initial patch started checking the existing code and
noticed that it was just broken. So insted I ended up with the
attachment. Please let me know if that works for you, and I'll include
it in my next push.

> At second thought adding explicit_bzero and reallocarray to
> AC_CHECK_FUNCS is much more convenient, right?

The problem is that these are public headers, where we cannot (or
should not) make use of build time detection via config.h, because
those are to be used on the installed system where the libc version
might be different to the use to compile the library.

Thanks,
Guillem
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