CMake Nightly Date Stamp
cmMakefile: Add a CreateSource method

The GetOrCreateSource searches the source file listing again, but some
callers know that it already didn't exist.
cmMakefile: Avoid excess source files

When there are no commands, a main_dependency is not required and when
there are also no dependencies, nothing is required.
cmMakefile: Defer dependency calculations
Merge topic 'dev/custom-target-performance' into next

6208c285 cmMakefile: Defer dependency calculations
dc2e26df cmMakefile: Avoid excess source files
d2803fba cmMakefile: Add a CreateSource method
49c830d5 CMake Nightly Date Stamp