This commit also adds a check to see that all files that
are part of an xi:include also have part of XML_FILES
and vice versa. It also fixes any applications where this
was not true.

Two parallell doc builds running fop (as same user) could both
update the fop font-cache at the same time. The cache is now
created as an artifact in the build process.

"./configure --enable-lock-counter" would modify the behavior
of the default emulator (usually SMP).

To make lock counting more accessible, change --enable-lock-counter
to build an additional emulator in the same way as:

  (cd erts/emulator && make lcnt)

(The next commit will make it easier to start the built lock-counter
emulator.)