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.)

* binarin/even-more-absolute-paths/PR-1103/OTP-13800:
  Use perl discovered by configure
  Don't make assumptions about build tools paths

Create correct links to datatypes outside of the wx application.

The added time for generating the doc/specs/specs_* files is unwelcome
(they are currently not used).

One more followup to https://github.com/erlang/otp/pull/1056 and
https://github.com/erlang/otp/pull/1023

This time it's about `/usr/bin/env` and `/bin/cp`:
- `/usr/bin/env` in `diameterc` was used to find the bootstrapped
  `escript` executable. Changed it to exlpicit call to `escript` in
  Makefile.
- `/usr/bin/env` and `/bin/cp` were referenced in documentation
  build/install process. Now full paths to this tools are injected using
  autoconf magic.