Bug#880781: sagemath build depends on removed libgd2*-dev provides

[Adrian Bunk]

Source: sagemath
Version: 8.0-8
Severity: serious

The following packages have unmet dependencies:
 builddeps:sagemath : Depends: libgd2-noxpm-dev but it is not installable or
                               libgd2-xpm-dev but it is not installable


Please update the build dependency to libgd-dev.