pharo: Compile with -DPharoVM

This is important. The VM was not compiled in "Pharo mode" and this
made certain primitives return different values to the Smalltalk side.

(Practically speaking I am surprised that the VM has been working
basically fine for me for weeks, but this resolves a problem with
adding filetree:// repositories with Monticello that failed in an
obscure way when the file permissions lookup primitive did not behave
as expected.)

See also:
https://pharo.fogbugz.com/f/cases/20217/Image-does-not-detect-incompatible-opensmalltalk-vm