Summary: CMake runs correctly from the command line, but can’t find library files installed with homebrew when run from VSCode.

Here are my includes that are giving trouble:

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Box.H>

Here is my CMakeLists.txt. In lines 4, 5, and 6, you can see my various efforts to get it to find FLTK.

cmake_minimum_required(VERSION 3.5)
project(HelloFLTK)

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

add_executable(HelloFLTK main.cpp)
target_link_libraries(HelloFLTK ${FLTK_LIBRARIES})

When run from the command line:

% cmake --build .
[ 50%] Building CXX object CMakeFiles/HelloFLTK.dir/main.cpp.o
[100%] Linking CXX executable HelloFLTK
[100%] Built target HelloFLTK

Here is my tasks.json. Looks like it’s invoking CMake, right?

{
    "version": "2.0.0",
    "tasks": [
        {
            "label": "build",
            "type": "shell",
            "command": "cmake",
            "args": ["-DCMAKE_BUILD_TYPE=Debug", "${workspaceFolder}"],
            "group": {
                "kind": "build",
                "isDefault": true
            }
        }
    ]
}

Results of running with Command-shift-B. Inadequate command line generated. But I can confirm that Command-shift-B is bound to “Tasks: Run Build Task”

 *  Executing task: C/C++: g++ build active file 

Starting build...
/usr/bin/g++ -fdiagnostics-color=always -g /Users/ken/vsc/fl/main.cpp -o /Users/ken/vsc/fl/main
/Users/ken/vsc/fl/main.cpp:1:10: fatal error: 'FL/Fl.H' file not found
#include <FL/Fl.H>
         ^~~~~~~~~
1 error generated.

Build finished with error(s).

% ls /opt/homebrew/opt/fltk/include/FL
Enumerations.H          Fl_JPEG_Image.H         Fl_Table_Row.H
Fl.H                Fl_Light_Button.H       Fl_Tabs.H
Fl_Adjuster.H           Fl_Line_Dial.H          Fl_Text_Buffer.H
. . . and many, many more.

So what is creating a different situation in Visual Studio code, and how do I help it find these include files?

The whole source file, for reproduceability:

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Box.H>

int main(int argc, char *argv[]) {
    Fl_Window *window = new Fl_Window(300, 180);
    Fl_Box *box = new Fl_Box(20, 40, 260, 100, "Hello, dear World!");
    box->box(FL_UP_BOX);
    box->labelfont(FL_BOLD + FL_ITALIC);
    box->labelsize(36);
    box->labeltype(FL_SHADOW_LABEL);
    window->end();
    window->show(argc, argv);
    return Fl::run();
}