[Bro] compiling bro with tcmalloc

[Matthias Vallentin]

> Might I ask what "less reliable tcmalloc support" entails?  Anything specific or am I just in for random crashes, etc.?

For example, we have encountered a Darwin (10.7) setup with MacPorts
where Bro just segfaults immediately as it starts, you could not even
get to the usage information. Similar segfault issues apparently also
exist for FreeBSD 8.2. This does not mean that tcmalloc does not work
with BSD in general, just some specific configurations turn out to be
complicated.

    Matthias