Be more explicit about build-tree-only dependencies.

Be more explicit about build-tree-only dependencies.

Use the BUILD_INTERFACE CMake generator expression to improve
self-documentation and to prevent build-tree-only dependencies from
propagating to exported targets. When libgromacs is a STATIC target,
even private dependencies can affect the exported target through
transitive usage requirements (e.g. compiler/linker flags). However,
we currently only use the `common` and `legacy_modules` targets for
build time header paths.

Ref #3288