make testing the CodeBlocks and Eclipse generators easier by not requiring the CMAKE_EDIT_COMMAND variable

Both generators use the CMAKE_EDIT_COMMAND variable to determine whether
they should add the edit_cache target, i.e. they don't add it if it's
ccmake, since this does not work inside the output log view of
Eclipse/Codeblocks. But instead of requiring the variable to be set they now
check it for 0 and handle this appropriately. This should help Dave getting
some testing for them :-)

Alex