Conditionally localize TEST_JOBS to avoid empty var.

Otherwise, when $jobs is undef, the environment's
TEST_JOBS comes into existence but has empty value,
which results in spurious warnings in Perl test suite.
Merge branch 'master' of git://github.com/tokuhirom/Perl-Build
Merge pull request #24 from jest/master

Conditionally localize TEST_JOBS to avoid empty var.