Define _GNU_SOURCE when including fenv.h

[jamesjer]

Fedora has 2 architectures, ppc64le and s390x, where fedisableexcept is needed. However, with glibc 2.37 at least, the fedisableexcept prototype is visible only if _GNU_SOURCE is defined. This is remarked on in the "glibc notes" section of the fedisableexcept man page on Linux systems.

Also, since fenv.h is a system header file rather than a local header file, use angle brackets with #include instead of quotes.

[glebbelov]

This has been partially accepted, as follows:

20241111
   fpinit.c in solvers.tgz and solvers2.tgz:  insert
     #define _GNU_SOURCE
before
     include "fenv.h"
to make fedisableexcept() visible on some systems (as someone
requested).  It is "fenv.h" rather than <fenv.h> because it was
sometimes necessary to supply a custom fenv.h, as indicated in
the comment

/* Some Intel Linux systems (e.g., S.u.S.E. 5.2) come with a suitable
fenv.h, */
/* but some (e.g., S.U.S.E. 6.1) do not.  This is for the latter
systems.  */

Let us know if anything else is necessary. The change will appear on netlib, and we will port it to this repository too.

[jamesjer]

That's fine. The _GNU_SOURCE part was the important part. Thank you!