Makefile: handle broken curl version number in version check

Makefile: handle broken curl version number in version check

curl 7.11.0 through 7.12.2 when built from their official release
archives will present a 5 digit version number instead of the documented
6 digits which breaks the version check in the Makefile.
Correct these broken version numbers on the fly when extracting them to
ensure the comparison works correctly.

[jc: shortened the new sed scripts a bit]

Signed-off-by: Tom G. Christensen <tgc@statsbiblioteket.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>