[CMake] Valgrind and CTest on OS X -- num-callers default failure

[kent williams]

Maybe I'm out on the bleeding edge, but...  I built the current .svn
for valgrind on OS X 10.5.7.  Then I tried to do a memory check with
CTest for Insight.

Every test failed with this error message:

./Testing/20090708-0100/DynamicAnalysis.xml:==81796== '--num-callers'
argument must be between 1 and 50

Now if I look in the CMake source, -num-callers is set for 100.

Since this doesn't fail on e.g. Linux, I have to assume that the
current SVN valgrind has lowered the upper limit.