endian: Make sure system headers can be included. Reported by Bruno Haible in <https://lists.gnu.org/archive/html/bug-gnulib/2024-05/msg00290.html>. * lib/endian.in.h (be16toh, be32toh, be64toh, htobe16, htobe32, htobe64) (le16toh, le32toh, le64toh, htole16, htole32, htole64): Don't define functions if the system has working versions. * m4/endian_h.m4 (gl_ENDIAN_H): Separate checks for stdint types and proper macro/function definitions. * modules/endian (Depends-on): Add include_next. Update module dependency conditions. (Makefile.am): Perform sed replacements on the header substitute.