[BUGFIX] TypoScript userFunc condition does not work with static methods

TypoScript allows to utilize functions (user-functions) to
integrate custom condition processing. However, this currently
only works with function defined in the global namespace - it
does not work with static methods.

Thus, something like this snipped won't work:
[userFunc = ConditionMatcherUserFunctions::isTrue(1)]

The solution is simple, replace function_exists() with a more
generic is_callable() check.

Fixes: #62918
Releases: master, 6.2
Change-Id: I260c73cac26dfd14efc67bc9f65ff39d6ba74da8
Reviewed-on: http://review.typo3.org/34087
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>