Alexander Richardson authored Jul 12, 2021 and Werner Lemberg committed Jul 12, 2021

Without this change the build of `unix/ftsystem.c` fails because the
`ftconfig.h` header that defines macros such as `HAVE_UNISTD_H` and
`HAVE_FCNTL_H` is only being generated for Linux, macOS, and Cygwin
systems:

```
.../builds/unix/ftsystem.c:258:32: error:
    use of undeclared identifier 'O_RDONLY'
file = open( filepathname, O_RDONLY );
```

Instead of hardcoding a list of operating systems for this check,
update the logic that decides whether to build the file and set a
boolean flag that can be checked instead.