When using this make file I wrote my program compiles however it doesn’t act the way it is supposed.

CC := g++
SRC := src
PROJECT_NAME := main
SRC_FILES := $(shell find $(SRC) -name '*.cpp')

inc := -I /usr/include/eigen3

CFLAGS := -Wall
OPTIMLEVEL := -O3


all:
    $(CC) $(CFLAGS) $(inc) $(SRC_FILES) $(OPTIMLEVEL) -o  $(PROJECT_NAME) 

.PHONY: clean
clean:
    rm -rf $(PROJECT_NAME)

running this simply results in an error

error: ‘A’ does not name a type A.diagonal().setConstant(4);

#include <Eigen/Dense>
int n = 4;
Eigen::MatrixXf A(n,n);

A.diagonal().setConstant(4);
A.setZero();
A.diagonal().setConstant(4);

int main (void){

    printf("Hello World! n");

    return 0;
}

I have checked and Eigen library lies in the stated directory. This is done in vscode and it provieds me with a diffrent error message.

#include errors detected. Please update your includePath. Squiggles are disabled for this translation unit (/home/albert/docs/hobby/Arduino/eigen_test/src/main.cpp).C/C++(1696)
cannot open source file “Eigen/Dense”