Define OMIT_UTMP if there's no utmpx.h.

Without this, the build of e.g. psusan would fail on systems without
that header (such as Termux on Android).

This is similar to how things were pre-cmake, but not identical. We used
to treat lack of updwtmpx() as a reason to OMIT_UTMP (as of f0dfa73982),
but usage of that function got conditionalised in c19e7215dd, so I
haven't restored that exclusion.

unix/pty.c

@@ -227,6 +227,8 @@ static void setup_utmp(char *ttyname, char *location)
     endutxent();
 
 #if HAVE_UPDWTMPX
+    /* Reportedly, AIX 5.1 has <utmpx.h> and pututxline(), but no
+     * updwtmpx(). */
     updwtmpx(WTMPX_FILE, &utmp_entry);
 #endif