In most cases it should not be necessary to configure anything before building.
If you do want to make configuration changes, you should start by going
to the xc/config/cf directory, and copying the file
xf86site.def to host.def. After that, read thoroughly
the the host.def file, which is heavily commented, and set
your configuration parameters. Usually you can find the default settings
by checking the .cf file(s) relevant to your OS.
A good rule of thumb is to only change that you understand, as it is
very easy to create build problems by changing the default configuration
unnecessarily. Before making too many modifications, check the
configuration parameters specified in the xc/config/cf/README
file.
If you are using just the XFree86-4.8.0-src-1.tgz,
XFree86-4.8.0-src-2.tgz and
XFree86-4.8.0-src-3.tgz parts of the source dist, you
will need to define BuildFonts to NO.