patch files using patch

For creating the OpenSpace AppImage, we need to apply some changes to the cfg file, and the most straightforward way to do it in the build script might be using patch.

https://www.baeldung.com/linux/shell-edit-patch-files

https://www.howtogeek.com/415442/how-to-apply-a-patch-to-a-file-and-create-patches-in-linux/

So we create a file with the needed changes and use diff to create the patch.

diff -u old/slang.c new/slang.c > slang.patch