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.