libctf: work on platforms without O_CLOEXEC.

(Not tested on any such platforms, since I don't have access to any at
the moment.  Testing encouraged.)

libctf/
	* configure.ac: Check for O_CLOEXEC.
	* ctf-decls.h (O_CLOEXEC): Define (to 0), if need be.
	* config.h.in: Regenerate.

libctf/ctf-decls.h

@@ -62,6 +62,10 @@ void ctf_qsort_r (void *base, size_t nmemb, size_t size,
 	      void *arg);
 #endif
 
+#ifndef HAVE_O_CLOEXEC
+# define O_CLOEXEC 0
+#endif
+
 #undef MAX
 #undef MIN
 #define MAX(a, b) ((a) > (b) ? (a) : (b))