Refactor everything to find and link libraries directly in INSTDIR.

- add gb_LinkTarget_get_linksearchpath_for_layer, and use it to set up
  -L paths for T_LDFLAGS in such a way that only allowed libraries
  can be linked against; i.e. it's not possible to link URE
  linktargets against OOO or not-installed libraries
- gb_Library_get_target is now same as the gb_LinkTarget_get_target
  (TODO: this needs cleanup)
- since a pattern rule won't work for linking libraries in INSTDIR,
  add a separate per-file rule for every INSTDIR lib
- pattern rule can't find link target in the clean target any more
  so add a LINKTARGET variable
- disable gb_Library_add_auxtarget, no auxtargets need to be copied
- tweak the call to gb_Library_Library_platform to pass in a path
  in sdk/lib for the versioned URE libs
- fix the Library clean target
- add LAYER parameter to gb_LinkTarget_LinkTarget
- adjust platform link commands
- MSVC link command now uses explicit -manifestfile and -pdb
  parameters to keep misc. files out of INSTDIR
- remove gb_Helper_OUTDIR_FOR_BUILDLIBDIR
- adjust Extension, CppunitTest,  JunitTest, PythonTest, Gallery,
  various CustomTargets to search INSTDIR
- remove SDK library symlinks and import libs from odk/Package_lib
- on Mac OS X, put .dylib symlinks into sdk/lib even though those
  are not packaged and would be created by the SDK configury;
  we need these to be somewhere for linking anyway
- add a (unfortunately cyclic) dependency on Package ure_install to sal

Change-Id: I70d88742f8c8232ad7b9521416275c67b64fe6cf