posix.c: Set errno when pipe2 is not available and flags provided.

If pipe2 is not available (e.g. on MacOS) and flags are set,
SCM_SYSERROR was correctly signaled, however errno was not set, so it
reported as:

    Undefined error: 0

That sucks both in tests (the test is not skipped) and in actual
usage (user has no idea what went wrong).

So set errno to ENOSYS as well.

* libguile/posix.c (scm_pipe2) [!HAVE_PIPE2] <c_flags>: Set errno to
ENOSYS.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

libguile/posix.c

@@ -282,7 +282,7 @@ SCM_DEFINE (scm_pipe2, "pipe", 0, 1, 0,
     /* 'pipe2' cannot be emulated on systems that lack it: calling
        'fnctl' afterwards to set the relevant flags is not equivalent
        because it's not atomic.  */
-    rv = ENOSYS;
+    rv = -1, errno = ENOSYS;
 #endif
 
   if (rv)