Don't recompile core.a if none of the .o files changed

Before, core.a would be rebuilt on every build, even when none of the
core .o files changed. Now, the timestamps are checked against the
timestamp on core.a first, skipping the build if nothing changed.

Because this uses the current list of .o files, there is a corner case
when a source file is deleted, but no other source file is modified. In
that case, core.a is not rebuilt, even though it should be. However,
this is such a narrow and unrealistic case, that it should pose a real
problem.

This fixes part of #1991