Merge mode: Duplicate symbol _glad_GL_loader_handle 
Command line:
```
--merge --api='gl:compatibility=4.6,gles2=3.2' --extensions='' c --header-only --loader
```

This invocation causes a duplicate symbol to be defined:  `_glad_GL_loader_handle`:

```
[ 33%] Building C object CMakeFiles/glad_egl_15_gles2_20.dir/gladsources/glad_egl_15_gles2_20/src/gl.c.o
/Users/kintel/code/OpenGL/glad/example/c/egl_x11/gladsources/glad_egl_15_gles2_20/src/gl.c:10134:14: error: redefinition of '_glad_GL_loader_handle'
static void* _glad_GL_loader_handle = NULL;
             ^
/Users/kintel/code/OpenGL/glad/example/c/egl_x11/gladsources/glad_egl_15_gles2_20/src/gl.c:9955:14: note: previous definition is here
static void* _glad_GL_loader_handle = NULL;
             ^
```