tools/libxc: Add Valgrind client requests

tools/libxc: Add Valgrind client requests

Valgrind client requests can be used by code to provide extra debugging
information about memory ranges, or to request checks at specific points.

Reference:
  http://valgrind.org/docs/manual/mc-manual.html#mc-manual.clientreqs

Client requests are safe to compile into code for running outside of
valgrind.  Therefore, enable client requests whenever autoconf can find
memcheck.h and debug builds are enabled.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- reran autogen.sh ]