The Single UNIX Specification requires the existence of unistd.h, which can tell you the POSIX version (via the _POSIX_VERSION macro).
But how can you include unistd.h if you don’t know yet that you are in fact compiling on a UNIX?
That is where this GCC document comes handy. According to it, testing for the presence, or evaluation-to-true of __unix__ should tell you that the system is a UNIX. So:
#ifdef __unix__
/* Yes it is a UNIX because __unix__ is defined. */
#include <unistd.h>
/* You can find out the version with _POSIX_VERSION.
..
.. */
#endif
__unix__ is not defined on Mac OS X, so to account for that, you could instead do:
#if defined (__unix__) || (defined (__APPLE__) && defined (__MACH__))
To get a list of system specific predefined macros on your system, you may execute:
cpp -dM /dev/null
For example, my GNU/Linux system also additionally defines __linux__ and __gnu_linux__ apart from __unix__ and a bunch of other stuff.
Another useful document that you must look at is this Wiki.
It goes on to present a way of detecting the presence and version of POSIX in a way similar to the one I described above.
EDIT: Since you really want to do all this because you want to decide which directory separator to use, look at this URL. It says:
Note File I/O functions in the Windows API convert “https://stackoverflow.com/” to “\” as part
of converting the name to an NT-style name, except when using the
“\?\” prefix as detailed in the following sections.
I don’t program on Windows, or know much anything about it, so I can’t say I’ve banked on this.