In general, I install QEMU on my Macbook using MacPorts.  However I recently had a need to get the tip of the QEMU development tree.

Getting the QEMU source tree is trivial:

git clone git://git.qemu-project.org/qemu.git

I needed an updated version of dtc:

git submodule update --init dtc

The build instructions from the README are:

 mkdir build
 cd build
 ../configure
 make

However, my case I only need ARM emulation, so:

../configure --target-list=arm-softmmu
make
make install

The binary qemu-system-arm will be at /usr.local/bin

oscar:build tom$ /usr/local/bin/qemu-system-arm --version
QEMU emulator version 2.4.94, Copyright (c) 2003-2008 Fabrice Bellard