As @dtlewis290 suggests, I think it's reasonable to look at my 585 changes as being the culprit. I changed Makefile.inc / acinclude.m4 to add _-I/usr/lib/x86_64-linux-gnu/glib-2.0/include_ when I saw that UnicodePlugin was failing to be configured on 64-bit Debian due to autoconf not finding a needed header. This resolved the issue on my Debian build VMs but these were not multiarch enabled (i.e. I had *either* a 32-bit or 64-bit build environment, not a combined one) so I suspect the problem might be with multiarch build environments having both 32-bit and 64-bit PangoCairo-related libraries installed at the same time? If that's the case, one solution could be to have the 'build for platform' detection append the include path to only the appropriate /usr/lib/<arch>/glib-2.0/include directory rather than both which should eliminate any conflict on multiarch enabled systems. If the issue isn't related to multiarch then I'm not sure what the cause is.