ExternalProject: Add EXCLUDE_FROM_MAIN option to ExternalProject_Add_Step

When adding a new step using ExternalProject_Add_Step, the main target
will depend on this step.
This option allows one to add a step that will not be executed when the
main target for the external project is executed.
ExternalProject: Add EXCLUDE_FROM_ALL option to ExternalProject_Add

When adding a new external project, the "all" target will depend on
this.
This option allows one to add an external project will not be executed
when the "all" target is executed.
The reason for this is that an external project could be useful, for
example, only for running tests, and therefore not necessary during
the build.
Merge topic 'ExternalProject_exclude-from-all' into next

1ddcc582 ExternalProject: Add EXCLUDE_FROM_ALL option to ExternalProject_Add
0a1c0129 ExternalProject: Add EXCLUDE_FROM_MAIN option to ExternalProject_Add_Step