[tlbuild] kpsesafe tests failing on macOS since last commit

[Andreas Scherer]

 > Maybe the symbol "unix" is not predefined by the mac compiler?
Indeed. Here's an article about predefined preprocessor macros for 
various C/C++ compilers: 
https://blog.kowalczyk.info/article/j/guide-to-predefined-macros-in-c-compilers-gcc-clang-msvc-etc..html 
(Similar info is available on stackexchange and web.archive.org.)

At least on macOS, "unix" is not among the output from "cc -x c 
/dev/null -dM -E" (Clang 15.0.0).

Apart from kpathsea/tex-file.c, there seems to be only dviljk/config.h 
with '#if defined (unix)'. The latter has
#elif defined (__APPLE__) && defined (__MACH__)
#define unix
to make the code work with Clang on macOS. '#if defined __APPLE__' (and 
'__MACH__') appears quite often in the TL source tree already.

Happy hacking!
Andreas