Fix EkatConfig.cmake so that the if statements are non-empty

[bartgol]

Motivation

When installing ekat, I noticed the following lines in EkatConfig.cmake:

  if () 
    find_dependency(yaml-cpp REQUIRED QUIET HINTS )
  else()

This is not valid cmake, as the if command requires at least one arg. The problem stems from the fact that we used <PKGNAME>_FOUND in EkatConfig.cmake.in, but these vars are LOCAL to the scope where find_package(...) was called. To overcome this, let's use some helper cache variables, which we set to ON/OFF depending on whether Ekat builds the tpl or not.

Testing

I verified that the generated EkatConfig.cmake file is indeed valid. As a TODO item, we may want to create a dummy cmake project (perhaps inside ekat's repo) that links against ekat. and that we can use in our CI to verify that the ekat installation logic works as expected.