configure must be re-run with all previous options as well

Otherwise defaults such as database type and database user may be
rewritten.
rt-test-dependencies doesn't look at $DisableGD, it needs --with-gd
Clarify that out of the box RT will handle Unicode in charts

Link to the %ChartFont option while I'm at it.
Minor word choice tweaks for clarity
Using July 1 and 31 is more clear, at least to me

Noting TIMTOWTDI is ancillary to showing people how to use charts.