libstdc++: Fix up FAIL in 17_intro/names.cc on glibc < 2.19 [PR108568]

libstdc++: Fix up FAIL in 17_intro/names.cc on glibc < 2.19 [PR108568]

On gcc112 which has glibc 2.17 I've noticed
FAIL: 17_intro/names.cc (test for excess errors)
FAIL: experimental/names.cc (test for excess errors)
These are because glibc < 2.19 used __unused as field member of various structs,
including mcontext_t in sys/ucontext.h on ppc64le.
This was changed in glibc with
https://gcc.gnu.org/pipermail/libc-alpha/2013-November/045766.html
names.cc even has
 #ifdef __GLIBC_PREREQ
 #if ! __GLIBC_PREREQ(2, 19)
 // Glibc defines this prior to 2.19
 #undef __unused
 #endif
 #endif
for it, but it doesn't work.  The reason is that __GLIBC_PREREQ is defined in
<features.h> but nothing included that header before this spot (it is included later
from bits/stdc++.h).

The following patch on Linux/Hurd conditionally includes features.h to get
the needed macros before deciding if __unused should be undefined or not.
If needed, I could use __GLIBC_PREREQ then but would need to check if it is
defined and between 1996 and 1999 it wasn't.

2023-01-27  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/108568
	* testsuite/17_intro/names.cc (__unused): For linux or GNU hurd
	include features.h if present and then check __GLIBC__ and
	__GLIBC_MINOR__ macros for glibc prior to 2.19, instead of testing
	__GLIBC_PREREQ which isn't defined yet.