[tlbuild] kpsesafe tests failing on macOS since last commit

[Bruno Haible]

Andreas Scherer wrote:
>  > 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.)

And on https://github.com/cpredef/predef (quite detailed).

Bruno