After much studying of cmake configuration, I got this program to build and run. Environment:
C++11
MacOS 14.2.1
FLTK library 1.3.9

But it draws nothing, the Fl_Gl_Window remains black. Any ideas? Recommendations for diagnostic steps? I have already tried changing the color from white to: glColor3f(0.5, 1.0, 1.0);
to no avail.

The code (based on a starter I got from chatgpt):

#define GL_SILENCE_DEPRECATION

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Gl_Window.H>
#include <FL/gl.h>
#include <OpenGL/glu.h>

void glutWireSphere (GLdouble radius, GLint slices, GLint stacks);

class MyGlWindow : public Fl_Gl_Window {
public:
    MyGlWindow(int x, int y, int w, int h, const char* l = 0) : Fl_Gl_Window(x, y, w, h, l) {}

    void draw() override {
        if (!valid()) {
            glLoadIdentity();
            glViewport(0, 0, w(), h());
            glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
            glEnable(GL_DEPTH_TEST);
            valid(1);
        }

        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();

        // Set up the camera
        gluLookAt(0.0, 0.0, 3.0,  // eye position
                  0.0, 0.0, 0.0,  // look-at position
                  0.0, 1.0, 0.0); // up direction

        // Draw a sphere
        glColor3f(0.5, 1.0, 1.0);
        glutWireSphere(1.0, 20, 20);

        glFlush();
    }
};

int main() {
    Fl_Window* window = new Fl_Window(400, 400, "FLTK Sphere Example");
    MyGlWindow* glWindow = new MyGlWindow(10, 10, window->w() - 20, window->h() - 20);
    window->resizable(glWindow);
    window->end();
    window->show();
    return Fl::run();
}

CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)
project(HelloFLTK)

set(CMAKE_PREFIX_PATH  "/opt/homebrew")
find_package(FLTK REQUIRED)
include_directories(${FLTK_INCLUDE_DIRS} /opt/homebrew/include)
link_directories(${FLTK_LIBRARY_DIRS})

add_executable(HelloFLTK flsphere.cpp)
target_compile_features(HelloFLTK PUBLIC cxx_std_11)
target_link_libraries(HelloFLTK ${FLTK_LIBRARIES})

BTW GL_SILENCE_DEPRECATION is needed because Apple, in their infinite wisdom, is threatening to get rid of OpenGL entirely. I am trying hard to resurrect and enjoy a piece of legacy code before that happens!