Small patch: --disable-setproctitle flag

Hello

Recently I discovered that renaming processes using setproctitle() call
on BSD systems may sometimes cause problems. For instance there is
currently a bug in all versions of LLDB which makes it impossible to
debug a process that called setproctitle():

https://llvm.org/bugs/show_bug.cgi?id=26924#c3

Since LLVM stack is used by default in FreeBSD I believe it's quite a
severe problem. In case there is other software that doesn't handle
stproctitle() well and for users of LLDB <= 3.8 (most recent version)
I propose to add a --disable-setproctitle flag to configure script.
Corresponding patch is attached.

--
Best regards,
Aleksander Alekseev
http://eax.me/