Warn when constant name is used for unexpected object

This fixes the sort of weird things like `fc-match :size=rgb` done without any errors.
This might be annoyed but the error messages should helps to fix an application bug or
suggest more useful constant names to fontconfig.

Fixes #137
2 jobs from master in 4 minutes and 59 seconds
Status Job ID Name Coverage
  Build
passed #53850
shared-build

00:04:59

passed #53851
static-build

00:04:28