To build android on x86 machine, you may mention to http://www.android-x86.org/
However, it seems Google does the simliar work silently.
You may following the document, build/target/board/generic_x86/README.txt
Before you update your repo, may I have your attention:
If you are a Mac user, get a Linux box first. No such darwin-x86 cross compiler exist in repo.
Following is the instruction
The generic_x86 board target provides basic services on very basic
hardware (really for an emulation). To build with generic_x86, you will
need an appropriate kernel for your emulation (or device).
A1. Create a new top level directory and pull the AOSP repository
repo init -u git://android.git.kernel.org/platform/manifest.git
A2. Copy in the kernel
cp ~/bzImage.your_device $HOME/AOSP/prebuilt/android-x86/kernel/kernel
The build will generate some image files whose format may or may not be correct for your
device. You can build an installer image disk for the VirtualBox emulator using the command:
A4. Build a VirtualBox installer image
make -j8 installer_vdi