The library is using EdwardsCurve25519 (which implementation and its operations are in assembly language).
Using these simple steps
make clean
phpize
./configure
make
sudo make install
won't allow me to compile the extension because in some of the assembly files, there are constants - which are expected to come from another assembly file or event from a .c file.
The issue is caused by PIC - because by default the code should be Platform Independent - but having some hardcoded constants or references to a constant from a different file is not platform-independent.