Installing libff results in awkward header structure

[etam]

This is almost duplicate of #16, but without libsnark.

Here's what I did:

git clone https://github.com/scipr-lab/libff.git
cd libff
git submodule update --init
mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX="${PWD}/install" ..
make -j4
make install

The result is that in path "install/include/libff" are header from libff, but no headers from "depends" are anywhere installed.