Add "The" to the start of monster death messages. Closes #804
Stop mulitdrop from being affected by changes to u.inv.size()
Make version a prerequisite for version.h instead of all.

The way it was could cause version.o to be built before version.h when multithreading was used for make.
Merge bugfixes from fergus-dall.
Fix some more makefile stuff

Remove version.h when doing a make clean
Ensure that version.o is rebuilt every time version.h changes
Make season length default to 14 days. Closes #671
Merge PR #821 (fix season length default) and #820 (makefile, version.h stuff)