get config_deps from prereqs if available

[kazeburo]

get config_deps from {meta}->{prereqs}->{configure} if available.
configure_requires is deprecated in spec 2

[miyagawa]

Looks great! Do you have a sample dist that fails with this?

[kazeburo]

sample dist
https://github.com/kazeburo/CPAN-Test-Dummy-ConfigDeps
Build.PL requires Module::CPANfile.

failed log

% cpanm --self-upgrade
--> Working on App::cpanminus
Fetching http://www.cpan.org/authors/id/M/MI/MIYAGAWA/App-cpanminus-1.6001.tar.gz ... OK
Configuring App-cpanminus-1.6001 ... OK
Building and testing App-cpanminus-1.6001 ... OK
Successfully installed App-cpanminus-1.6001 (upgraded from 1.5021)
1 distribution installed
% perl -MModule::CPANfile -e ''
Can't locate Module/CPANfile.pm in @inc (@inc contains: /.....).
BEGIN failed--compilation aborted.
% cpanm -L extlib -n .
--> Working on .
Configuring CPAN-Test-Dummy-ConfigDeps-0.01 ... N/A
! Configure failed for CPAN-Test-Dummy-ConfigDeps-0.01. See /home/kazeburo/.cpanm/build.log for details.