The simple answer would be that the libgit project folder is not where the build expects to find it. You are going to want a folder structure like:
The libgit2 folder might be a symbolic link to a more specific version folder.
I do dev builds using qtcreator, but the same folder structure is needed either way. “dev-area” is whatever folder you want to use.
libgit needs its own build before starting the Fritzing build. Verify that you have read the main build notes and the extra for linux. That might be easier to follow for you than it was for me. You are building on Debian, I used Fedora, so I had to tweak some of the dependencies.