Merge: nitc: use ccache again

c02e04291807cf97228068cc5b52f453d84ab37b unexpectedly disable ccache.
Since CC is set by default [1], the ?= attempt did never assign anything [2].

The solution is to assign unless if CC is set by another mean.
This can be done thanks to the origin function [3].

  [1]: https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html
  [2]: https://www.gnu.org/software/make/manual/html_node/Flavors.html
  [3]: https://www.gnu.org/software/make/manual/html_node/Origin-Function.html

Pull-Request: #2497