[patch] Fix bug 15915, erroneous make rule for linkobj/libc.so.

[Brooks Moses]

As described in bug 15915, the pattern rule for lib%.so files in
Makerules includes linkobj/libc.so as a dependency.  However, the
explicit rule for linkobj/libc.so is in the top-level Makefile.

Thus, the subdirectory makefiles that include Makerules end up with an
erroneous makefile pattern rule for linkobj/libc.so that includes
itself as a dependency.  The result is make warnings whenever rules
for other .so files are resolved -- and, on occasion, actual makefile
failures when a race condition causes the implicit rule to actually be
used.

This patch moves the explicit rules for linkobj/libc.so into Makerules
to clear up this problem.  It also elaborates a couple of comments
that I'd initially found confusing.

I've confirmed that this removes the circular-dependency warnings, and
that the build still completes correctly.

Ok for trunk?

- Brooks
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