I'm not using cups but need the hpijs server so set USE=minimal. Build stops with 'includefile not found: cups/cups.h'
I can workaround with simply 'mkdir /usr/include/cups ; touch /usr/include/cups/cups.h' ie presenting an empty file
Works on my setup, so please provide emerge --info and build.log and emerge -vp net-print/hplip
Reopen this bug when you provide the requested information.