Previously ECLIPSE_CDT4_GENERATE_SOURCE_PROJECT was used, but the
new name CMAKE_ECLIPSE_GENERATE_SOURCE_PROJECT is more in line with
the general naming conventions in cmake, and, more importantly IMO,
in cmake-gui it now appears right next to the other eclipse-related
variables, which all start with CMAKE_ECLIPSE_.

A warning is printed if the old variable is TRUE and the new one isn't,
so users should notice that they have to enable the new one.

Alex