Hello Chiara,
I had met the same question with you, then I checked my cmake and I found there was something wrong with my cmake.
So I reinstall cmake , when I run "./bootstrap" it said " version `GLIBCXX_3.4.20' not found" .Then I search for solution ,
and the key factor is libstdc++, you can input this "strings /usr/local/lib64/libstdc++.so.6|grep GLIBCXX" if you found the"GLIBCXX_3.4.20" you could put the link to right address as following:
% cp /usr/local/lib64/libstdc++.so.6.0.21 /usr/lib64/
% cd /usr/lib64/
% rm -f libstdc++.so.6
% ln -s libstdc++.so.6.0.21 libstdc++.so.6
maybe you need a root permission, you can try it!