Change how we create the 'ganeti' symlink

Currently, if one runs 'make' in an already fully-built tree, this is
the result:

  cd . && test -h "ganeti" || { rm -f ganeti && ln -s lib ganeti; }
  make  all-am
  make[1]: Entering directory `/tmp/test'
  cd . && test -h "ganeti" || { rm -f ganeti && ln -s lib ganeti; }
  make[1]: Leaving directory `/tmp/test'

This is because commit dc7d2c49 added 'ganeti' (which is a PHONY
target) to BUILT_SOURCES, and since that is a dependency of other,
real targets, it means the ganeti target is always remade.

To fix this, we keep ganeti as a PHONY target, but we remove it from
the 'built_base_sources' target, and instead we only remake it
manually in the stamp-directories target. A make run now is just:

  make  all-am
  make[1]: Entering directory `/tmp/test'
  make[1]: Nothing to be done for `all-am'.
  make[1]: Leaving directory `/tmp/test'

Note that we can't get rid of the all-am since we use BUILT_SOURCES.

We also remove the comment of BUILT_SOURCES since it no longer depends
on PHONY targets.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>