Mark Module::Build as deprecated from Perl core

Per these discussions, Module::Build is deprecated from core in v5.19
and will be removed in v5.21:

* http://www.nntp.perl.org/group/perl.perl5.porters/2013/05/msg202041.html
* http://www.nntp.perl.org/group/perl.perl5.porters/2013/05/msg202463.html

This commit uses the 'deprecate' module to issue a warning if run from a
core library path.  Module::Build installed from CPAN will not warn.