unknown type name 'PFNGLACCUMXOESPROC'
I'm helping a person setting up GLAD for their CMake-Project. Now on build  (using CLion using mingw64) the following error occurs:
```
[1/3] Building C object CMakeFiles/glad_gl_core_33.dir/glad-build/src/gl.c.obj

FAILED: CMakeFiles/glad_gl_core_33.dir/glad-build/src/gl.c.obj 

C:\msys64\mingw64\bin\cc.exe -I"C:/Users/alpha/3D Objects/backroomsC/external/glfw-3.3.8/include" -I"C:/Users/alpha/3D Objects/backroomsC/external/glfw-3.3.8/deps" -I/glad-build/include -g -MD -MT CMakeFiles/glad_gl_core_33.dir/glad-build/src/gl.c.obj -MF CMakeFiles\glad_gl_core_33.dir\glad-build\src\gl.c.obj.d -o CMakeFiles/glad_gl_core_33.dir/glad-build/src/gl.c.obj -c /glad-build/src/gl.c

/glad-build/src/gl.c:658:1: error: unknown type name 'PFNGLACCUMXOESPROC'; did you mean 'PFNGLACCUMPROC'?

 658 | PFNGLACCUMXOESPROC glad_glAccumxOES = NULL;

   | ^~~~~~~~~~~~~~~~~~

   | PFNGLACCUMPROC

/glad-build/src/gl.c:658:39: warning: initialization of 'int' from 'void *' makes integer from pointer without a cast [-Wint-conversion]

 658 | PFNGLACCUMXOESPROC glad_glAccumxOES = NULL;

   |                    ^~~~
```
(Error is way longer, but that's how it starts.)

We've verified that glext.h is present in the include folder of mingw64 and that the include folder is included by gcc.

CMakeList.txt looks like the following:
```
cmake_minimum_required(VERSION 3.23)

set(PROJECT_NAME backroomsC)

project(${PROJECT_NAME})

add_executable(${PROJECT_NAME} main.cpp util/List.h util/List.cpp)
set(SOURCE_FILES main.cpp) # welche Dateien kompiliert werden sollen

set(GLFW_SOURCE_DIR  "external/glfw-3.3.8/src")
set(GLAD_SOURCES_DIR "${CMAKE_SOURCE_DIR}/external/glad")

set(GLFW_LIBRARY_TYPE STATIC)
set(GLFW_BUILD_EXAMPLES OFF)
set(GLFW_BUILD_TESTS OFF)
set(GLFW_BUILD_DOCS OFF)
set(GLFW_INSTALL OFF)
set(CMAKE_STATIC_LIBRARY_SUFFIX ".lib")
set(CMAKE_SHARED_LIBRARY_SUFFIX ".dll")

find_package(OpenGL REQUIRED)
if(NOT OPENGL_FOUND)
    message(ERROR "OPENGL not found!")
endif(NOT OPENGL_FOUND)
if(OPENGL_INCLUDE_DIR STREQUAL "")
    message(ERROR "OPENGL_INCLUDE_DIR is empty!")
endif(OPENGL_INCLUDE_DIR STREQUAL "")

add_subdirectory(external/glfw-3.3.8)

include_directories(${OPENGL_INCLUDE_DIR})

include_directories(external/glfw-3.3.8/include external/glfw-3.3.8/deps)

add_subdirectory("${GLAD_SOURCES_DIR}/cmake")
glad_add_library(glad_gl_core_33 REPRODUCIBLE LOCATION "/glad-build" API gl:core=3.3)

target_link_libraries(${PROJECT_NAME} glad_gl_core_33 ${GLFW3_LIBRARY} glfw ${OPENGL_LIBRARIES})
```

Any idea what might went wrong?