I have problem in newest ns-3.26 with nsc-0.5.3 and gccxml-ns3 - system is very up-to-date so that's not the case.
Building nsc-0.5.3 - Problem
Problem: Optional dependency, module "nsc-0.5.3" failed
This may reduce the functionality of the final build. However, bake will continue since "nsc-0.5.3" is not an essential dependency. For more information call bake with -v or -vvv, for full verbose mode.
Found the solution - NSC doesn't work properly with gcc/g++-6 so I had to change default version of gcc and g++ (for example from 6.3.0 to 5.4.1)
1) Find installed compilers:
dpkg --list | grep compiler
2) Change default gcc, g++, cc and c++ (via https://askubuntu.com/questions/26498/choose-gcc-and-g-version):
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 10
sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-5 20
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 10
sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-5 20
sudo update-alternatives --install /usr/bin/cc cc /usr/bin/gcc 30
sudo update-alternatives --set cc /usr/bin/gcc
sudo update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++ 30
sudo update-alternatives --set c++ /usr/bin/g++
Also, to check if changes are saved:
gcc -v
It should say: "gcc version 5.4.1" at the end.
After that, NSC was installed successfully.
It's worth mentioning that it didn't fix gccxml's problem, but in my case I needed NSC working so...