kdesrc-build does not respect the CMAKE_GENERATOR environment variable.
The latest CMake release added the CMAKE_GENERATOR environment variable to set a default generator for CMake to use. kdesrc-build does not seem to specify the generator used explicitly but does expect it to be Unix Makefiles which breaks kdesrc-build when the CMAKE_GENERATOR variable is set to for example Ninja.
A possible fix could be to check if the CMAKE_GENERATOR is and use its generator if it is set.