check dependencies for libraries.
Because these targets (e.g. minimime/libmmime.a) are real ones,
declaring them .PHONY would cause them to be rebuilt every time
(see e.g. SVN 64355).

As a workaround I am using the following CHECK_SUBDIR target:

  CHECK_SUBDIR:  # do nothing, just make sure that we recurse in the subdir/
  minimime/libmmime.a: CHECK_SUBDIR
	@cd minimime && $(MAKE) libmmime.a

which seems to do a better job than .PHONY (probably because
.PHONY forces the rebuild even if the recursive make does not think
it is necessary).

If this turns out to be the correct approach, we can then
merge it back into 1.4



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@72700 65c4cc65-6c06-0410-ace0-fbb531ad65f3