When I want to use valgrind to check for leaks in a Python program (or
test suite), I generally want pymalloc disabled.  When not running
valgrind I generally want it enabled.

Attached is a patch that automatically bypasses the pymalloc code when
running under valgrind but leaves it enabled overwise.

It is controlled by a WITH_VALGRIND #define, but I haven't updated the
configure script to allow turning it on.  I also haven't done much in
the way of profiling to see what the overhead is when not running under
valgrind.