No content or form changes for the xcb manual or tutorial.
Only the configuration user visible bits change.

Xcb will now have the same configuration options as the 30 other
xorg modules.

Xorg classifies documentation as "user", "developer" or "specifications".
The xcb manual falls under the "developer" category. Developers docs
are never installed under $prefix.

A builder can selectively turn on/off any or all of the categories. He can
also selectively turn on/off any of the many tools used to generate
documentation such as doxygen, xmlto, etc... Each tool has an environment
variable defined such as DOXYGEN.

Other features are available, the user interface and the functionality
is the same on all modules.

--with-doxygen=FILE is replaced with DOXYGEN env variable
--disable-build-docs is replaced with --disable-devel-docs

The new interface displayed with ./configure --help:

  --enable-devel-docs     Enable building the developer documentation
                          (default: yes)
  --with-doxygen          Use doxygen to regenerate documentation (default:
                          auto)
  DOXYGEN     Path to doxygen command
  DOT         Path to the dot graphics utility

The dot tool checking has been added to util-macros in version 1.18.

Refer to the table of existing docs in xorg.
XCB will be added for the doxygen generated API manual.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>