cpan.bbclass: Fail unless Makefile.PL generates Makefile

cpan.bbclass: Fail unless Makefile.PL generates Makefile

Makefile.PLs can exit with success without generating a Makefile,
e.g. in cases of missing configure time dependencies. This is
considered a best practice by cpantesters.org. See:

 * http://wiki.cpantesters.org/wiki/CPANAuthorNotes
 * http://www.nntp.perl.org/group/perl.qa/2008/08/msg11236.html

(From OE-Core rev: 0a2dffefc6fbca37ce9604117cebc2f7d4abc1c6)

Signed-off-by: Olof Johansson <olof.johansson@axis.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>