PATCH: move Valgrind header checks from "valgrind" to "misc" checking

[Laurynas Biveinis]

PING

http://codereview.appspot.com/4250047
http://gcc.gnu.org/ml/gcc/2011-01/msg00363.html

this patch moves Valgrind header detection from "valgrind" checking to "misc"
and enables
the latter whenever the former is enabled.

If only "misc" is enabled, then Valgrind header presence is optional.

I plan to followup with another patch that adds new configure option
--enable-valgrind-annotations that is orthogonal to checking and causes
configure to fail
if headers are not present.  Also I will update wwwdocs once this is accepted to
trunk.

-- 
Laurynas