gitweb: Skip 'modification times' tests when no date parser available
The If-Modified-Since support in Gitweb is conditional on the
availability of a date parser from either the HTTP::Date or
Time::ParseDate modules. If a suitable parser is not available,
then the corresponding 'modification times' tests should be skipped.
Introduce the DATE_PARSER test prerequisite and use it to skip
all of the dependent tests.
Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>