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 commit 84e7920b (FindFontconfig: Add module to
find Fontconfig, 2018-09-27, v3.14.0-rc1~523^2).

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.
Reported-by: Christophe Giboudeaux <christophe@krop.fr>
Fixes: #19094