$ pwd
/home/albrecht/git/flruler/build/wayland
$ rm -rf *
$ cat ../../CMakeLists.txt 
cmake_minimum_required (VERSION 3.2.3)

project (flruler)

# find FLTK ...
# FLTK_DIR *MUST* be defined on the commandline or in the GUI

set (FLTK_DIR "${CMAKE_BINARY_DIR}/fltk-1.4"
      CACHE FILEPATH "FLTK installation or build directory")

# (2) Find FLTK

find_package (FLTK REQUIRED NO_MODULE)

# (3) Debug: show found variables ...

if (0)
  message (STATUS "--- Project: ${CMAKE_PROJECT_NAME} ---")
  message (STATUS "FLTK_INCLUDE_DIRS     = '${FLTK_INCLUDE_DIRS}'")
  message (STATUS "FLTK_LIBRARIES        = '${FLTK_LIBRARIES}'")
  message (STATUS "FLTK_FLUID_EXECUTABLE = '${FLTK_FLUID_EXECUTABLE}'")
  message (STATUS "FLTK_USE_FILE         = '${FLTK_USE_FILE}'")
endif ()

# flruler executable

set (SOURCES flruler.C)

add_definitions (-DVERSION=\"1.05\")
add_executable (flruler ${SOURCES})
target_include_directories (flruler PRIVATE ${FLTK_INCLUDE_DIRS})

target_link_libraries (flruler fltk)
$ cmake -G Ninja -DFLTK_DIR="/home/albrecht/git/fltk/wayland/build/debug" ../..
-- The C compiler identification is GNU 10.2.1
-- The CXX compiler identification is GNU 10.2.1
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/albrecht/git/flruler/build/wayland
$ ninja 
[2/2] Linking CXX executable flruler
$ ninja clean 
[1/1] Cleaning all built files...
Cleaning... 2 files.
$ ninja -v
[1/2] /usr/bin/c++ -DVERSION=\"1.05\" -I/home/albrecht/git/fltk/wayland/build/debug -I/home/albrecht/git/fltk/wayland -I/usr/include/cairo -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/uuid -I/usr/include/freetype2 -I/usr/include/libpng16  -MD -MT CMakeFiles/flruler.dir/flruler.C.o -MF CMakeFiles/flruler.dir/flruler.C.o.d -o CMakeFiles/flruler.dir/flruler.C.o -c ../../flruler.C
[2/2] : && /usr/bin/c++  -rdynamic CMakeFiles/flruler.dir/flruler.C.o -o flruler  /home/albrecht/git/fltk/wayland/build/debug/lib/libfltk.a  -ldl  -lpthread  -lpango-1.0  -lpangoxft-1.0  -lpangocairo-1.0  -lcairo  -lgobject-2.0  -lXft  -lfontconfig  -lwayland-egl -lEGL  -lwayland-cursor -lwayland-client -lxkbcommon -ldl -ldbus-1  -lgtk-3  -lgdk-3  -lpangocairo-1.0  -lpango-1.0  -latk-1.0  -lcairo-gobject  -lcairo  -lgdk_pixbuf-2.0  -lgio-2.0  -lgobject-2.0  -lglib-2.0  -no-pie && :
$ ./flruler
$