Move the byte-order related wrappers out of the individual source
files into a dedicated header file. Modify the single hand-coded
source file that uses the byte-order wrappers to use the new header
file.
- Use autoconf tests instead of platform-specific #ifdef's to decide
which macros to use.
- Provide fallbacks for platforms like Solaris that don't provide any
of the existing known forms.
defining __GLIBC__, as done already for Linux and Hurd (cf. bug #5613).
Also includes some more __GNU__ checks on top of those made in
commit ade4bf0907.
Thanks Petr Salinger (Debian bug #400869).
