Undefined reference to 'gladLoadGLLoader'
I have a pre-compiled GLFW3 shared library and want to use it with glad to build a really basic OpenGL dev environment. Basically, I'm following [this tutorial](https://learnopengl.com/Getting-started/Creating-a-window) on **Ubuntu 18.04**. The `glad.c` is located within my project tree whereas `glad.h` is located in a separate folder under my HOME directory together with the GLFW headers. My CMake file looks more or less like this:
``` cmake
cmake_minimum_required(VERSION 3.8)

project(opengl LANGUAGES CXX)

set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} "${CMAKE_SOURCE_DIR}/CMake/Modules/")

find_package(GLFW3 3.3 REQUIRED)

set(EXT_SOURCE_DIR "${CMAKE_SOURCE_DIR}/external/src")

add_executable(${PROJECT_NAME} "${EXT_SOURCE_DIR}/glad.c" "${CMAKE_SOURCE_DIR}/src/main.cpp")

target_include_directories(${PROJECT_NAME} PUBLIC "${EXT_INCLUDE_DIR}/glad" "${EXT_INCLUDE_DIR}/KHR" ${GLFW3_INCLUDE_DIR})

target_link_libraries(${PROJECT_NAME} ${CMAKE_DL_LIBS} ${GLFW3_LIBRARY})
```
CMake builds this simple project whose 'main.cpp' basically contains [this code](https://learnopengl.com/code_viewer_gh.php?code=src/1.getting_started/1.1.hello_window/hello_window.cpp). However, `make` throws `main.cpp:(.text+0xee): undefined reference to 'gladLoadGLLoader'` as the first error. 
Actually, I don't really understand why something like `find_package(OpenGL)` is not needed in several glad examples, but even if I use this together with `${OPENGL_LIBRARIES}` in the `target_link_libraries` it throws the same error.