bazelbuild
Allow cmake variable to be set to the empty string
The cmake() tules interprets an entry in the cache_entries dictionary with a value of "" as meaning that the key, as a cmake variable name, should be unset. I have a case where I need to actually set a cmake variable to "" because the target library's CMakeFiles.txt checks to see if a variable is set and if not sets it to its idea of a default. This change would allow the user of the cmake() rule to set a variable to "" by specifying the dictionary value as "
This is working for me but I'm open to other approaches. I think it'd be better to instead remove the existing interpretation of "" and then add a new list to the cmake() rule that contains the variables to leave unset in the final set passed to cmake. This would probably break existing users, though.
