A makefile for downloading and building libgsf from source

make

In the makefile:

• Update the VERSION variable to the relevant version
• Check that the C_FILES and H_FILES variables are correct and match the contents of the zip that is downloaded

Once complete add a tag on master:

git checkout <commit to release>
git tag <version to release>
git push origin <version to release>

Then the publish action will pick up the tag and create a release