In most cases it shouldn't be necessary to configure anything before building.
If you do want to make configuration changes, it is recommended that
you start by going to the xc/config/cf directory, and copying
the file xorgsite.def to host.def. Then read through
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 to follow is only to change things that you understand
as it's easy to create build problems by changing the default configuration.
Many of the configuration parameters are described in the xc/config/cf/README.
If you are using just the X11R6.9.0-src1.tar.gz,
X11R6.9.0-src2.tar.gz and
X11R6.9.0-src3.tar.gz parts of the source dist, you
will need to define BuildFonts to NO.