[CMake] INSTALL documentation does not mention default component Unspecified

[Bill Lorensen]

If an INSTALL command does not provide a COMPONENT, it seems that a
component named "Unspecified" is created. This is not mentioned
anywhere in the cmake documentation as far as I can tell.

This cmake file illustrates the problem:

cmake_minimum_required(VERSION 2.8)
project(bug)

add_executable(myTarget a.cxx)
target_link_libraries(myTarget -lm)
INSTALL (
    TARGETS myTarget
    EXPORT myExportedTargets
    RUNTIME DESTINATION bin
    LIBRARY DESTINATION lib
    ARCHIVE DESTINATION bin
  )

get_cmake_property(PROPS COMPONENTS)
message(STATUS "COMPONENTS: ${PROPS}")