Since lib{expr,store,util}-test-support subprojects define nix_api_* helpers
for testing nix c bindings, they need to publicly depend on -c counterparts.
This makes their headers self-sufficient and does not rely on the -tests to add
necessary dependencies.
This reduces the amount of boilerplate. More importantly, it provides
a place to add compiler flags (such as -O3) without having to add it
to every subproject (and the risk of forgetting to include it).
