[CMake] Adding files to ARCHIVE packages only in CPack

[Bruno Barberi Gnecco]

	How is it possible to add some files only to the ARCHIVE generators with CMake/CPack? 
Apparently components do that, but I can't figure how to say "only add component X to 
generator Y". I did something like this:

	In CPackConfig.cmake:

set(CPACK_PROJECT_CONFIG_FILE 
"${CMAKE_CURRENT_SOURCE_DIR}/CMakeModules/CMakeCPackOptions.cmake")

	In CMakeCPackOptions.cmake

IF (CPACK_GENERATOR MATCHES "TGZ")
	install(FILES myextrafile DESTINATION "." COMPONENT static)
ENDIF()

	But I get the following error:

/usr/local/bin/cpack --config ./CPackConfig.cmake
CMake Error at CMakeModules/CMakeCPackOptions.cmake:4 (install):
   Unknown CMake command "install".

CPack Error: Cannot initialize the generator TGZ

	What am I doing wrong? Any other way to run `install()` only for certain generators? Thanks