The only complicating factor here is having to split out the --target
flag from cargo_build_flags, so we can pass the appropriate one
depending on our build target.
This new ChromeManifestEntry object type is generic and can hold any kind of
chrome manifest entry, but we currently only emit them for binary components.
References to sub-directory manifests is left to the backend, for now, until
all manifest entries are emitted by the frontend.
The recursivemake backend sets IMPORT_LIBRARY to the same value as
SHARED_LIBRARY on non-Windows platforms, so we can simply use
IMPORT_LIBRARY everywhere.
NO_INSTALL_IMPORT_LIBRARY is only used in one place, and since we don't even
use $(DIST)/lib for gecko, it actually doesn't make a difference presently.