lang/ghc92: Use ${_PKGSRC_MKPIE} instead of ${_OPSYS_SUPPORTS_MKPIE}

It's the variable that indicates if MKPIE is requested and feasible on the
platform. In fact I originally wrote a condition mentioning
${_PKGSRC_MKPIE}, but it somehow got lost while I was refactoring the code.

(pho)