Use statement-expressions without warnings, even in strict ISO C mode.
Suggested by Eric Blake <eblake@redhat.com> in
<https://lists.gnu.org/archive/html/bug-gnulib/2023-09/msg00025.html>.
* lib/error.in.h (__gl_error_call): Use the variant with obvious control
flow also with clang. Use '__extension__' to avoid -Wpedantic warnings.
* lib/math.in.h (gl_signbitf, gl_signbitd, gl_signbitl): Define as
macros even when __STRICT_ANSI__ is defined. But use '__extension__' to
avoid -Wpedantic warnings.
* lib/setenv.c (KNOWN_VALUE): Use '__extension__' to avoid -Wpedantic
warnings.
* lib/xalloc-oversized.h (xalloc_oversized): Use optimized variant even
when __STRICT_ANSI__ is defined. But use '__extension__' to avoid
-Wpedantic warnings.