How can I include a external dependency source c file

I am trying to start opengl develop with cMake on Qt, the base informations are as below:
glad: version4.6; glfw: version3.3.4, QT: version5.12.
the file tree as below:

my cmake are as below:
cmake_minimum_required(VERSION 3.5)

project(opengl1 LANGUAGES CXX)


set(GLFW_3P ${PROJECT_ROOT_DIR}/3rdpart/glfw)
set(GLAD_3P ${PROJECT_ROOT_DIR}/3rdpart/glad)
set(GLAD_FILE ${GLAD_3P}/src/glad.c)

    ${GLAD_FILE})  #**acttually, this file add failed**

    PUBLIC ${GLFW_3P}/include
    PUBLIC ${GLAD_3P}/include
find_library(GLFW_LIB glfw3 HINTS {GLFW_3P}/lib)

target_link_libraries(opengl1 ${GLFW_LIB})

but when compile, all the functions located in glad.c are “undefined reference to”, the file glad.c not invove compile:
cmd.exe /C "cd . && C:\Qt\Tools\mingw810_64\bin\g++.exe -g src/CMakeFiles/opengl1.dir/main.cpp.obj -o src\opengl1.exe -Wl,--out-implib,src\libopengl1.dll.a -Wl,--major-image-version,0,--minor-image-version,0 C:/openGl/glfw/lib/libglfw3.a -lkernel32 -luser32 -lgdi32 -lwinspool -lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32 && cd ."

Q: how can I include a external c file successfully? (thank you so much if get you response)

Is it possible that your *.c files are simply getting ignored because you only have enabled CXX language?

hi my friends, thank you so much for your big help, this fucking issue had takes me three days, your advice is useful. When you come to Beijing, I’ll treat you to dinner!!!, thank you again

Sometimes you just need a fresh eye.

Glad I could help.