This is going to be used to allow DIRDEPS to be bootstrapped off of
LIBADD/DPADD. It currently works for internal libraries which have a
DIR defined for them but also use the .a library from a src-mapped obj
directory. It can also be useful for using -L without a --sysroot per
LIBADD to use the OBJDIR version of the libraries.
I didn't review every LIBADD, so it is possible this is missing some.
Sponsored by: EMC / Isilon Storage Division
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