Incidentally, this makes the build fail because stdc++compat.o then appears
twice on the libxul linkage command line. This, in turn, is because
widget/xremoteclient creates both a program and an intermediate library for
libxul. The Program() template adds stdc++compat to the directory data,
which ends up being added to libxul as well. The same kind of issue arises
when linking the gtest-enabled libxul.
While eventually we'll be able to avoid those problems, it's not the case
yet, so work around the issue by making expand-libs skip .desc files that
appear multiple times.
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