OK, now I understand: an ‘=’ in the right-hand side of a dependency in a Makefile define a target-dependent variable, whose name is the left-hand side of the equal sign and the value is the right-hand side of the equal. This variable will be valid only when executing commands to build the current target (what is on the left of the ‘:’) or one of its dependencies.
The problem there was that K:/ is definitly not a valid variable name... If it had been, you will not have had the error, but the build will have failed for another reason, as a dependency would have missed for your target...
So, it’s right: avoid fancy folder names, and especially always omit ‘=’ signs in a folder name (by the way its the first time I see such a folder name...)