A font editor.
Developer Documentation: http://fontforge.github.io/en-US/documentation/developers/ (source)
For building from git, and for a more-or-less definitive list of dependencies and versions required, see
travis.yml, which gives all the commands needed for a Debian-based system; on other GNU systems and non-GNU systems, you will need to use your system’s package manager to install the dependencies, or install them manually. Fontforge supports Python >= 2.6, but to run the tests you need Python 3. Read more...