You need to install GCC8 and G++8. If you dont put the correct version of gcc it will cause an error with the library filesystem. Check with cmake --version that you have the correct version of the compiler.

$ export CXX=/usr/bin/g++-8
$ export CC=/usr/bin/gcc-8
$ git clone https://github.com/AlexandreRouma/SDRPlusPlus.git
$ mkdir build && cd build
$ cmake ..
$ make CC=gcc-8