Detect the endianness with CMake

What is the bug

The endianness detection in src/util_hash.c produces a false result on macOS with GCC 13 or later. For whatever reason, #include <sys/param.h> does not define the __DARWIN_BYTE_ORDER and __DARWIN_LITTLE_ENDIAN macros when #define _POSIX_C_SOURCE 200112L. And that leads to the wrong values of the HASH_LITTLE_ENDIAN and HASH_BIG_ENDIAN macros. See also https://gitlab.dkrz.de/icon/icon-mpim/-/merge_requests/684#note_300951.

How do you fix it

The endianness detection is delegated to CMake.

How urgent is the bugfix

• I need it as soon as possible
• I can wait for a couple of days
• None of my current codes is directly affected

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• Bugfix is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Test coverage does not decrease
• Reviewed by a maintainer
• Incorporate review suggestions
• Prior to merging, please remove any boilerplate from the MR description, retaining only the What is the bug and How do you fix it section to maintain
• Remember to edit the commit message and select the proper changelog category (feature/bugfix/other)

You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!