Eigen issues: (1) it fails to find Eigen headers (2)

[yurivict]

(1) This error:

In file included from ../NeuralAmpModelerCore/NAM/activations.cpp:1:
../NeuralAmpModelerCore/NAM/activations.h:6:10: fatal error: 'Eigen/Dense' file not found
    6 | #include <Eigen/Dense>
      |          ^~~~~~~~~~~~~
1 error generated.

is because CXXFLAGS is missing pkg-config --cflags eigen3.

(2) Build fails with this error:

n file included from ../NeuralAmpModelerCore/NAM/get_dsp.cpp:8:
../NeuralAmpModelerCore/NAM/lstm.h:26:84: error: no member named 'lastN' in namespace 'Eigen::placeholders'
   26 |   Eigen::VectorXf get_hidden_state() const { return this->_xh(Eigen::placeholders::lastN(this->_get_hidden_size())); };
      |                                                               ~~~~~~~~~~~~~~~~~~~~~^

Not sure why lastN isn't found.