libctf: look for BSD versus GNU qsort_r signatures

We cannot just look for any declaration of qsort_r, because some
operating systems have a qsort_r that has a different prototype
but which still has a pair of pointers in the right places (the last two
args are interchanged): so use AC_LINK_IFELSE to check for both
known variants of qsort_r(), and swap their args into a consistent order
in a suitable inline function.  (The code for this is taken almost
unchanged from gnulib.)

(Now we are not using AC_LIBOBJ any more, we can use a better name for
the qsort_r replacement as well.)

libctf/
	* qsort_r.c: Rename to...
	* ctf-qsort_r.c: ... this.
	(_quicksort): Define to ctf_qsort_r.
	* ctf-decls.h (qsort_r): Remove.
	(ctf_qsort_r): Add.
	(struct ctf_qsort_arg): New, transport the real ARG and COMPAR.
	(ctf_qsort_compar_thunk): Rearrange the arguments to COMPAR.
	* Makefile.am (libctf_a_LIBADD): Remove.
	(libctf_a_SOURCES): New, add ctf-qsort_r.c.
	* ctf-archive.c (ctf_arc_write): Call ctf_qsort_r, not qsort_r.
	* ctf-create.c (ctf_update): Likewise.
	* configure.ac: Check for BSD versus GNU qsort_r signature.
	* Makefile.in: Regenerate.
	* config.h.in: Likewise.
	* configure: Likewise.

libctf/ctf-archive.c

@@ -169,10 +169,11 @@ ctf_arc_write (const char *file, ctf_file_t ** ctf_files, size_t ctf_file_cnt,
       modent++;
     }
 
-  qsort_r ((ctf_archive_modent_t *) ((char *) archdr
-				     + sizeof (struct ctf_archive)),
-	   le64toh (archdr->ctfa_nfiles),
-	   sizeof (struct ctf_archive_modent), sort_modent_by_name, nametbl);
+  ctf_qsort_r ((ctf_archive_modent_t *) ((char *) archdr
+					 + sizeof (struct ctf_archive)),
+	       le64toh (archdr->ctfa_nfiles),
+	       sizeof (struct ctf_archive_modent), sort_modent_by_name,
+	       nametbl);
 
    /* Now the name table.  */