Merge branch 'leak-detective-dlsym'

Replaces the use of deprecated glibc malloc hooks by overloading malloc calls
and find the originals using dlsym(RTLD_NEXT).

On OS X, we now support the use of leak detective by changing the default
malloc zone functions, backtraces get resolved using atos.

It seems that the performance bottleneck now is stack unwinding. Unfortunately
a new libunwind based backtrace() is not much faster than the libc variant; we
keep that option nonetheless for platforms without backtrace().