Support Rust compilation with -l. Fixes #88, #123.
Support Rust compilation with -l. Fixes #88, #123.

The previous commit made sccache refuse to cache rustc invocations that
generate anything other than rlibs or staticlibs. For rlibs and staticlibs
we can safely cache Rust compilations that pass -l, because the system
linker is not involved.

For static libraries, we need to include the contents of the static libraries
in the hash calculation, since they will get included wholesale into the
output rlib.

mozilla/sccache,glandium/sccache,luser/sccache2,mozilla/sccache,luser/sccache2,glandium/sccache,luser/sccache2,mozilla/sccache,luser/sccache2,glandium/sccache,glandium/sccache