fix cmake shader dependency issue

cosets/CMakeLists.txt

@@ -1,18 +1,18 @@
 project(cosets)
 
+add_custom_target(shaders ALL DEPENDS shader_output)
+add_custom_command(
+        OUTPUT shader_output
+        COMMAND ${CMAKE_COMMAND} -E copy_directory ${CMAKE_CURRENT_SOURCE_DIR}/src/shaders ${CMAKE_CURRENT_BINARY_DIR}/shaders
+        COMMENT "copying shaders"
+)
+
 add_executable(${PROJECT_NAME}
         src/main.cpp)
-
+add_dependencies(cosets shaders)
 target_link_libraries(${PROJECT_NAME}
         PRIVATE glad glm glfw)
 
-add_custom_command(
-        TARGET ${PROJECT_NAME} POST_BUILD
-        COMMAND ${CMAKE_COMMAND} -E copy_directory
-        ${CMAKE_CURRENT_SOURCE_DIR}/src/shaders
-        ${CMAKE_CURRENT_BINARY_DIR}/shaders
-)
-
 add_executable(coxeter src/coxeter.cpp)
 
 add_executable(mirror src/mirrors.cpp)