So, according to your explanation and the Lintian issues [1], the synfig package maintainer should update Build-depends list:
- add Build-Depends: libmagick++-dev (>= 7:6.4.2)
- change Build-Depends: libgl1-mesa-dev => libgl-dev (>= 1.7.0-1+b1)
- change Build-Depends: libfontconfig1-dev => libfontconfig-dev(>= 2.15.0-1.1)
- change Build-Depends: libfreetype6-dev => libfreetype-dev (>= 2.13.2+dsfg-1+b4)
Also:
- change Depends: libfontconfig1-dev => libfontconfig-dev(>= 2.15.0-1.1)
- change Depends: libfreetype6-dev => libfreetype-dev (>= 2.13.2+dsfg-1+b4)
And for synfigstudio [2]:
- change Build-Depends: pkg-config => pkgconf (>= 1.8.1-1+b2)
- Recommends: python3