FindFontconfig: Convert module variables to camel case

Our documented standard for find module variable names is to match the case of the find module package name. This was overlooked when the module was added by !2487 (merged).

The module was released with the upper case names in CMake 3.14.0, so fix it to have camel case names in 3.14.1. This is incompatible but anyone using a given release series should be using the latest patch on it and we've made breaking fixups on newly released features like this before.

Fixes: #19094 (closed)