Probe for libcheck by default.

Probe for libcheck and build checks (if found) by default.
Can be explicitly disabled using --disable-check-utests.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

configure

@@ -169,7 +169,7 @@ mixemu="no"
 aix="no"
 blobs="yes"
 pkgversion=""
-check_utests="no"
+check_utests=""
 user_pie="no"
 zero_malloc=""
 trace_backend="nop"