more subtle dependencies for cross-compilation

Hi there,

with 
http://cgit.freedesktop.org/libreoffice/core/commit/?id=bdfdd054bd25714f9c52c974f22228bee4bfbe67
cross-compiling should be faster because we don't build so much (maybe
- I did not test how much).

What could be interesting is that in solenv/gbuild/extensions/pre_BuildTools.mk
we have a list of tools we need for building.
Their run-time dependencies should be specified mostly in
RepositoryExternal.mk I hope.
I've tested this with disabled help and dictionaries - so there is
something missing probably.

Also there is new gb_Module_add_targets_for_build you can use instead of
ifneq ($(CROSS_COMPILING),YES) in Module_foo.mk
It's turned off when cross-compiling in
solenv/gbuild/extensions/post_SpeedUpTargets.mk

I guess some modules could be removed in RepositoryModule_build.mk,
because we don't need to parse its makefiles but it should be much better now.

Best,
Matus