[REVIEW 3-5][PATCH] Fix leap year calculation

[Kohei Yoshida]

Hi there,

The attached patch fixes a bug in our current leap year calculation
code.  It is based on the algorithm posted on wikipedia[1], and seems to
correctly identify year 2000 as a leap year.

Without this, Calc would convert 2000-2-29 into 1899-12-30 on load,
which is ugly but very hard to detect.

Please consider this for 3.5.1, in which case I'll need 3 sign-offs.  I
haven't pushed this to master yet.  I wanted to run it by Eike first
before pushing to master.

Kohei

[1] http://en.wikipedia.org/wiki/Leap_year

-- 
Kohei Yoshida, LibreOffice hacker, Calc