ENH: Add documentation for allowing only a specific target to be built

CTEST_BUILD_TARGET is a CTest special variable that accepts the name of a
specific build target, to be built instead of the implied "all".

CTEST_BUILD_TARGET only works if CTEST_BUILD_COMMAND is not defined (which
it typically isn't for ITK dashboard builds).

Change-Id: I3f54f38e20e7d476b763d82be428a7e8a498c971