ghostscript: link against "systemwide" liblcms2

GS ships with a fork of lcms2 ("lcms2mt"), but the ABI separation
between the fork and the original seems insufficient. If libgs is linked
alongside liblcms2 (for example, this is the case with imagemagick) then
it will call into the original library instead of the fork, causing
segfaults.

Follow the example of both Arch and Debian in this regard -- they both
use the systemwide lib instead of the fork.