`src/data/map_group_count.h` is currently in `.gitignore`. However, git
does not ignore a file if that file is already in the index. To actually
ignore the file, the file must be deleted and that deletion must be
committed.
In addition, the Makefile did not previously clean `src/data/map_group_count.h`,
nor know how to generate `src/data/map_group_count.h`.
It did generate the file as a side-effect of a different rule, but if
asked to generate, it did not know how to.
Currently, when Make is building the maps, it considers every
folder in the data/maps/ directory as a map that needs to be built,
even folders that are empty. This can be a problem when modding on
a separate branch, because git doesn't always clean up directories
when switching branches (usually because of hidden files in those
directories), so switching back to an unmodded branch will leave
an empty folder that confuses make.
This commit changes the makefile so that it only checks directories
that have map.json files in them.
