Y_SITE prefix is used to install the three key files (yplot.so, yplot.i, and
70yplot.i) that implement the yplot extension of yorick. If that option is
turned OFF (the default) those three key files are installed using the
${CMAKE_INSTALL_PREFIX}/lib/yorick prefix instead. The advantage of the
latter course is you don't have to have access to the root account to
install the yplot extension to Yorick (depending on how you have set
CMAKE_INSTALL_PREFIX). The disadvantage is you must use Yorick's add_y_home
function to add ${CMAKE_INSTALL_PREFIX}/lib/yorick to Yorick's normal Y_SITE
to enable the yplot extension of Yorick.
