Create a mechanism for providing fine-grained build order dependencies
during SUBDIR_PARALLEL builds.  This augments the coarse .WAIT mechanism,
which is still useful if you've got a situation such as "almost everything
depends on A and B".

Because the parallel subdir mechanism uses non-obvious mangling of
target names, which should probably remain a private detail of the
implementation, it's not easy to do things like "libfoo: libbar", so
instead the new mechanism lets you set a variable that lists dependencies:

  SUBDIR_DEPEND_libfoo= libgroodah libpouet

Note that while I'm using libraries as an example here, it really has
nothing to do with the generated library files.  This is really saying
"build in directory libfoo after building in the libgroodah and libpouet
directories."

This updates lib/Makefile with dependency information based on the old
almost-accurate comment block and by combing through lib/* makefiles
looking for LDADD dependencies to other libraries within lib/*.

Reviewed by:	Jia-Shiun Li <jiashiun@gmail.com>