MacOS - changes to the core code to allow DLite to compile on MacOS

[nameloCmaS]

Description

To allow compiling on MacOS, this update includes:

• Pre-processor updates.
• Switch changes for code handling files.

See #1060 for proposed changes.

Commentary per file

byteorder.h.in

• Added generated comment at top of file as a warning once processed.
• Including error in preprocessor HAVE__BYTESWAP_* (missing double underscore).
• Added MacOS equivalents of endian.h and byteswap.h plus defines to align naming of functions.

getopt.h

• Moved #ifndef _GETOPT_H_ after #include statement as original position prevented the loading of getopt.h (at least on MacOS).

config.h.in

• Added HAVE_ defines that were set in the CMake config but unused in the codebase.
• Uncommented have statements as these were set in the CMake config.

Remaining files

• Pre-processor updates.
• Switch changes for code handling files.
• Updated doxygen comments where function name was incorrect in fileutils.c.

Type of change

• Bug fix & code cleanup
• New feature
• Documentation update
• Test update

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[jesper-friis]

Bug fixes and consistency improvement.