Added explicit forward dependencies for libraries

Allow a forward dependency for a library to be specified like so:

$(eval $(call library,mylib,...,laterlib))
$(eval $(call library_forward_dependency,mylib,laterlib))
...
$(eval $(call library,laterlib,...))

It works because after the $(call libary,mylib...) is eval'd, its so name is in LIB_mylib_SO, and so the dependency can be fully specified.