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>