[mkgmap-dev] Commit r4600: fix wrong option handling introduced with r4598

Version mkgmap-r4600 was committed by gerd on Mon, 25 Jan 2021 fix wrong option handling introduced with r4598 If --dem option is given but no --overview-dem-dist, the overview map was created with high detailed dem data http://www.mkgmap.org.uk/websvn/revision.php?repname=mkgmap&rev=4600