Your build environment must be FreeBSD 10.0-RELEASE/FreeBSD 10.1-STABLE (building on FreeBSD 9 or 11 is not supported at this time).
an amd64 capable processor. 8GB of memory, or an equal/greater amount of swap space, is also required
You will need the following ports/packages when compiling anything FreeNAS-related:
- ports-mgmt/poudriere-devel
- devel/git
- sysutils/cdrtools
- archivers/pxz
- lang/python (2.7 or later, with THREADS support)
- sysutils/grub2-pcbsd
- sysutils/xorriso
- www/npm
- devel/gmake
- py-sphinx
- py-sphinx_rtd_theme
- py-sphinxcontrib-httpdomain
(and all the dependencies that these ports/pkgs install, of course)

Building the System Quickstart Flow:

Checking out the code from git:

% cd /path/to/your-build-filesystem
% git clone git://github.com/freenas/freenas.git --branch freenas10/development
% cd freenas

Build it

% make git-external
% make checkout
% make release

Update the source tree, to pull in new source code changes

% make update

This will also fetch TrueOS and ports for the build from github.

The End Result:

If your build completes successfully, you'll have both 32 and 64 bit release products in the release_stage directory. You will also have a tarball in your build directory containing the entire release for easy transport.