Turn cast from "const char *" to "char *" from implicit (which in the
psttf.cc case ended up as a build error) to explicit form. This build
error was found as a result of running the comprehensive_test.sh
script, but as far as I can tell nothing was changed for psttf.cc from
when this script ran without errors not that long ago for exactly
the same (normal build as opposed to epa_build) environment. So I
do not have a clue why we suddenly have this build error, but I
have now fixed it with the explicit casts.