function_exists checks in utf8 lib for compatibility with 3rd party libs

The DokuWiki UTF-8 library and its derivates are very popular in Open Source
PHP software. This makes trouble due to name clashes, when 3rd party
software libraries need to be loaded within DokuWiki.

A common example is using the authentication libraries of popular forum
systems (PHP3, PunBB3).

With the checks added, DokuWiki will rely on the 3rd party UTF-8 functions
instead of its own ones. As long as they are really the same, this will
work.

Users of 3rd party libs need to check compatibility between implementations
themselves.

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