Bug 1036645 part 1 - Use AC_OUTPUT_SUBDIRS to call ICU configure. r=gps

As ICU needs an objdir different from its source directory, add support for
that in AC_OUTPUT_SUBDIRS.
Bug 1036645 part 2 - Make using a separate config.cache the default for AC_OUTPUT_SUBDIRS. r=gps

All subconfigures are essentially doing it already, so just inverse the process. That would also limit problems with additional subconfigures (all the recent ones had to come with their own config.cache)
Bug 1036645 part 3 - Fix the subconfigure wrapper heuristics to decide when to kill config.cache. r=gps
Bug 1036794 - Remove MOZ_APP_COMPONENT_LIBS. r=mshal
Bug 1036864 - Remove EXPORT_LIBRARY. r=mshal