add support to prove for sane perl devel-env.
add checks to prove for pureperl build

As mentioned in the Lancaster Consensus, there're settings any Makefile.PL
or Build.PL should care to make a decision to prevent XS building. These
settings are handled appropriate to allow easy "with xs" checks for
progressive module authors.
add some comfort for checking perl api can be used
add latest changelog entries and prepare release