pstoedit: depends on xz

`/usr/lib/liblzma.dylib` not provided by 10.8. See https://github.com/Homebrew/homebrew-science/issues/1026

```
clang++ [snip] -lfreetype -llzma -lbz2 -lz -lltdl ./.libs/libpstoedit.dylib -lstdc++ -ldl -lm  -O2   -pthread
Should be guarded e.g. `if MacOS.version < mavericks`