Reported by Paul Eggert in
<https://lists.gnu.org/archive/html/bug-gnulib/2024-05/msg00249.html>.
* lib/byteswap.in.h (_GL_BYTESWAP_HAS_BUILTIN_BSWAP16)
(_GL_BYTESWAP_HAS_BUILTIN_BSWAP32)
(_GL_BYTESWAP_HAS_BUILTIN_BSWAP64): Define using the GCC version or
__has_builtin after checking that it is defined.
(bswap_16, bswap_32, bswap_64): Use the macros.
* modules/byteswap (Depends-on): Add stdbool as a conditional
dependency.