To reproduce:

Load this URL (in Firefox - I've not tested on Chrome or anything else) which is for glad 2 headers for OpenGL 4.6 compatibility with Options "mx" and "header only":

http://glad.sh/#api=gl%3Acompatibility%3D4.6&extensions=&generator=c&options=HEADER_ONLY%2CMX

Now, in "Options" uncheck "mx".

Press "Generate"

Compare the generated file gl.h with:

https://github.com/ABRG-Models/morphologica/blob/359-windows-gl-headers/morph/glad/gl.h

I found them to be the same apart from the generation date.

If I then load

http://glad.sh

And manually choose OpenGL 4.6 Compatibility with only the option "Header Only", in this case, the generated file DOES differ from my original.