Deal with recent perl default removal of '.' from @INC

[celane]

just need a definition in the %build section, harmless if perl doesn't remove '.'
(see: http://blogs.perl.org/users/todd_rinaldo/2016/11/what-happened-to-dot-in-inc.html)

Pretty sure that other 'build perl module' packages have this problem as well, but
when they fail I prefer to fall back to simpler tools like cpanspec.

diff -up ./cpanspec-1.78/cpanspec.orig ./cpanspec-1.78/cpanspec
--- ./cpanspec-1.78/cpanspec.orig 2020-12-26 10:40:41.443813026 -0500
+++ ./cpanspec-1.78/cpanspec 2020-12-26 10:42:08.183698533 -0500
@@ -1083,6 +1083,7 @@ END
print $spec <<END;

%build
+declare -x PERL_USE_UNSAFE_INC=1
END

 if ($usebuildpl) {