what systems need executable libraries?

["Jeremy C. Reed" <reed%reedmedia.net@localhost>]

I recall discussion about some systems need libraries to be installed with 
execute bit. pkgsrc checks for it and most (all?) of pkgsrc was fixed to 
do that.

Where is this documented? What systems need executable libraries?

Also why does INSTALL_PROGRAM have -s but INSTALL_LIB does not?