[TYPO3] Getting PATH_tslib from within a module?

[Kasper ligaard Nielsen]

I have installed the EXT:mwimagepath, but when accessing the BE-module I 
get an error, since 'tslib' is hardcoded.
I run Typo3 4, where the tslib path has been moved.

My question is if it is possible to access PATH_tslib (or something 
equivalent) from a backend module? I.e. to make the extension work on 
both Typo3 4 and earlier.

regards, Kasper