meson: fix Perl version check for Meson versions before 1.7.0

meson: fix Perl version check for Meson versions before 1.7.0

Command `perl --version` says, e.g., “This is perl 5, version 26,
subversion 0 (v5.26.0)”, which older versions of Meson interpret as
version 26.

This will be fixed in Meson 1.7.0, but at the time of writing that isn’t
yet released.

If we run `perl -V:version` we get the unambiguous response
“version='5.26.0';”, but we need at least Meson 1.5.0 to be able to do that.

Note that Perl are seriously considering dropping the leading 5 entirely
in the near future (https://perl.github.io/PPCs/ppc0025-perl-version/),
but that shouldn’t affect us.

Signed-off-by: Peter Oliver <git@mavit.org.uk>
Co-authored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>