libctf: suppress spurious failure of malloc-counting tests under valgrind

The libctf-regression/open-error-free.c test works by interposing malloc
and counting mallocs and frees across libctf operations.  This only
works under suitably-interposable mallocs on systems supporting
dlsym (RTLD_NEXT, ...), so its operation is restricted to glibc
systems for now, but also it interacts badly with valgrind, which
interposes malloc itself.  Detect a running valgrind and skip the test.

Add new facilities allowing libctf lookup tests to declare themselves
unsupported, by printing "UNSUPPORTED: " and then some meaningful
message instead of their normal output.

libctf/
	* configure.ac: Check for <valgrind/valgrind.h>.
	* config.h.in: Regenerate.
	* configure: Likewise.
	* testsuite/lib/ctf-lib.exp (run_lookup_test): Add support for
	UNSUPPORTED tests.
	* testsuite/libctf-regression/open-error-free.c: When running
	under valgrind, this test is unsupported.

libctf/testsuite/lib/ctf-lib.exp

@@ -270,6 +270,11 @@ proc run_lookup_test { name } {
 	set results [run_host_cmd "$opts(wrapper) tmpdir/lookup" $lookup_output]
     }
 
+    if { [regexp {^UNSUPPORTED: (.*)$} $results -> reason] } {
+	unsupported "$testname: $reason"
+	return 0
+    }
+
     set f [open "tmpdir/lookup.out" "w"]
     puts $f $results
     close $f