Actually, setting SAGE_GCC_INSTALL=yes didn't help.
Also, installing the recommended rpms
libffi
libffi-devel-3.2.1.git259-11.1.x86_64 didn't help initially.
However, in order to install the rpms, I had to remove libffi-devel-gcc5, which broke some dependencies. I also was supposed to install som libffi7. Which I did.
Now I have installed:
libffi-devel-3.2.1.git259-11.1.x86_64
libffi4-32bit-5.3.1+r233831-10.1.x86_64
libffi4-5.3.1+r233831-10.1.x86_64
and ecl compiles, miraculously!
I'm not sure, whether this is of any help.
I'm also not sure, what the consequences of this broken dependency will be. gcc5 doesn't show up in my rpm-list any longer.
But at least for now I'm happy.
Thanks a lot for your help,
Johannes.