Ignore all user files from QtCreator

If you open the project in QtCreator again after having made changes
to CMakeLists.txt outside of it, it may create a new user file which
should be ignored just like the original.
Tip for enabling IDE debugging in QtCreator
Enable release mode from wrapper script

By using --disable-debug (--enable-debug is the default), release mode
can be toggled from the wrapper script.

Some IDEs, notably QtCreator requires this flag to be set when running
CMake, in order to enable debugging properly. Thus, it is explicitly
set (and printed) when using ./configure also.