If you have the proprietary NVIDIA driver installed and you get the following error when executing the build target above:
/usr/bin/ld: cannot find -lGL
collect2: ld returned 1 exit status
scons: *** [build/release/radiant/radiant.bin] Error 1
scons: building terminated because of errors.
then you should install one of the nvidia-glx-dev* packages as mentioned above in Step A, then try to execute the main build target to compile GtkRadiant again.