Conditional directives for strerror_r don't work with musl

[nightlark]

Describe the bug
Using musl instead of glibc results in the #else branch in systemErrorText getting used; musl follows the POSIX standard.

To Reproduce
Compiling a program using ghc::filesystem on Alpine Linux (uses musl as its libc by default) as part of a CI build is how I encountered this issue. Otherwise, setup another compiler to use musl instead of glibc.

Expected behavior
The variant of strerror_r that returns an int should be used.

Additional context
Line 208 of the log in https://cloud.drone.io/nightlark/HELICS-src/22/2/4 is where I first saw the problem.

[gulrak]

Well, musl has the XSI-compliant version of strerror_r wrapped with an irritating:

#if defined(_POSIX_SOURCE) || defined(_POSIX_C_SOURCE) \
 || defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
 || defined(_BSD_SOURCE)

which doesn't seem match the man-file canonic test of (_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE because of the _GNU_SOURCE difference.
I decided to use a different approach instead of keeping to try to find a perfect set of macro tests, as I guess there will be more distributions with unusual choices in this function. Android is also special in this regard.

Still need to run a few tests, but expect something on a feature branch soon, I plan to make a "bugfix" release on the weekend.

[nightlark]

Yea, getting rid of the macro is the change I tried -- it took care of the compile errors with musl.

[gulrak]

Yeah, should be much more stable over the platforms, to simply dispatch. And all filesystem tests on the CI tested platforms are green, so I'll merge the change into master.

[gulrak]

Fix is now part of release v1.2.2.