gettext-full: force compiling target in C23 standard

gettext-full: force compiling target in C23 standard

Currently, the build system overrides the value of the CC variable
for actual compilation after configuring for target builds.
However, the configure script now modifies the CC variable
to include "-std=gnu23" when the test for C23 features is successful.
The configure script also tests for the ability to use alignof
without including the stdalign.h header, and only includes it if necessary.
The test in the configure script is being done with the C23 standard option
but the compilation is being done without the C23 standard option,
leading to an unusual build error where alignof() is not defined.

Resolving the conflict between the autoconf macros and the build system
causes several other packages to fail, so instead in the meantime,
force the use of C23 standard to compile as part of the new standard
includes alignof as a keyword to deprecate the stdalign.h macro.

Forcing use of the new standard is safe for target builds
as the toolchain is known to support the option
and is always within our scope of version control.

Signed-off-by: Michael Pratt <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19748
Signed-off-by: Robert Marko <[email protected]>