Ninja Multi-Config: Add support for cross-config custom commands

Help/command/add_custom_command.rst

Help/command/add_custom_target.rst

Help/generator/Ninja Multi-Config.rst

Help/manual/cmake-generator-expressions.7.rst

Help/release/dev/custom-command-output-genex.rst

Source/cmCustomCommandGenerator.cxx

Source/cmCustomCommandGenerator.h

Source/cmGeneratorTarget.cxx

Source/cmGlobalNinjaGenerator.cxx

Source/cmGlobalNinjaGenerator.h

Source/cmLocalGenerator.cxx

Source/cmLocalGenerator.h

Source/cmLocalNinjaGenerator.cxx

Source/cmLocalNinjaGenerator.h

Source/cmNinjaUtilityTargetGenerator.cxx

Source/cmNinjaUtilityTargetGenerator.h

...unCMake/FileAPI/codemodel-v2-data/targets/custom_tgt.json