perl: Set build date to SOURCE_DATE_EPOCH

Perl uses an internal #define called PERL_BUILD_DATE as the string for
the date when perl was built. If undefined, it defaults to using
__DATE__ and __TIME__, which is not reproducible. If SOURCE_DATE_EPOCH
is set at do_configure time, use it to generate a reproducible string
for PERL_BUILD_DATE.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>