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 fontconfig/fontconfig#137
2 jobs for <span class="ref-name">strict-const-name-in-prop</span> in 5 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #53837
shared-build

00:05:39

passed #53838
static-build

00:04:17