This commit introduces the ability to add custom targets
that correspond to individual ExternalProject steps.
The main motivation behind this new feature is to drive
sub-project based dashboard steps as external projects
with separate targets for update, configure, build and
test output. This makes it easy to construct a ctest -S
script to drive such a dashboard.

With no STEP_TARGETS defined, there are no additional
custom targets introduced, to minimize the chatter in
the target name space. Clients may define STEP_TARGETS
on a per-ExternalProject_Add basis, or at the directory
level by setting the EP_STEP_TARGETS directory property.