CMake allows you to "build" in directory somewhere else.
Typically, you build a cmake project as:
That last line means, "create the make files used for the build here, from the source directory above".
By default, this is what kdevelop uses, a subdirectory called "build".
To change it:
* open the project in kdevelop
* choose "Configure CMake settings"
* in the right pane, select "Show Advanced"
* at the bottom of the new options, "Show advanced values"
* in the upper pane, scroll to <project-name>_BINARY_DIR
That shows the absolute path of the build directory, change that.