Fix Makefile target order for parallel build

[nfetisov]

• Replace MY:postamble() with MY::depend()
  (fix: Parallel build rules dependancy problem #7)

[rschupp]

I think your analysis in #7 is correct, however I don't like this fix. It relies on a certain order of sections in the Makefile generated by ExtUtils::MakeMaker that could change.
Besides: if you override a method like depend() you should respect its interface - e.g. what would happen if someone added a "depend" argument to the call of WriteMakeFile().
Fixed in 04a133b by using adding a .NOTPARALLEL target.