CMake Nightly Date Stamp
ExternalProject: Add option to always run the build step

Teach ExternalProject_Add a new BUILD_ALWAYS option to skip using
the build step stamp file and execute the step on every build.

Extend the BuildDepends test with a case to cover this option.
Merge topic 'ExternalProject-BUILD_ALWAYS' into next

73e5c6ae ExternalProject: Add option to always run the build step
c0bbefbf CMake Nightly Date Stamp