CMP0026: Use compatibility codepath until configure is finished (#15748)

Use it instead of the similar cmMakefile API.  It is necessary to
know that the Configure step is completely done, not just that one
particular cmMakefile is finished configuring.

Prior to commit 611220f7 (cmTarget: Use reliable test for CMP0024 and CMP0026
OLD., 2015-07-25), this was determined by checking whether cmGeneratorTargets
exist yet, which happens after the Configure step.