Part for fontconfig fails to build against another part for freetype

I’m trying to build a snap for a GUI application which requires more recent versions of packages than provided for by Xenial. I am attempting to compile parts for FreeType and Fontconfig to support the application but failing.

Fontconfig seems unable to find FreeType’s ft2build.h file which is staged at usr/include/freetype2/ft2build.h. I’ve whittled down a test case scenario which shows the problem snapcraft.yml.

Could someone take a look at my snapcraft.yml, and attempt to build it, to see where I might be able to improve this or let me know whether it is broken in Fontconfig upstream or if this is a Snapcraft issue?