Merge branch 'timeout_cb_override' of git://github.com/nmathewson/timeout into nmathewson-timeout_cb_override

Merge branch 'portable_bitops' of git://github.com/nmathewson/timeout into nmathewson-portable_bitops

According to the documentation, 32-bit x86 builds do not have the
64-bit _BitScan{Forward,Reverse}64 intrinsics.

Given these, we can remove the code to check for the zero case.

This patch includes an intrinsic implementation for these functions
when the compiler is GCC, Clang, or MSVC--or if the compiler claims
to be one of those.  Otherwise, a naive implementation is used.

Tests are included for all of these functions, which turned up a
possible problem: according to the gcc documentation, __builtin_ctz
and __builtin_clz give an undefined result when their inputs is
zero.  I was not able to persuade myself that we always called them
with a nonzero argument.

I've found that frequently a void function and a pointer are either
more or less than we want to attach to each timeout object.  This
patch makes it so that the user can define TIMEOUT_CB_OVERRIDE to
indicate that they have included their own definition of struct
timeout_cb.

remove main() testing code from timeout.c and tweak Makefile so we can compare different wheel sizes with the benchmark framework

re-add one of our old bit fill operations, as it turns out it was actually needed (probably shouldn't have worked on that code in a sleep-deprived state)

add version routines and TIMEOUT_PUBLIC prefix to exported routines so applications can optionally compile completely statically (by defining TIMEOUT_PUBLIC to `static'