`make install` fails after ./configure --with-dlopen=no

This occurs with current HEAD (master) and probably in 9.14 though I have not checked that.

The problem is that the filter-aaaa.@SO@ target is broken when --with-dlopen=no because @SO_LD@ is undefined.

This is OK for the build stage, because ${TARGETS} is empty, so filter-aaaa.@SO@ is not built.

But the install target depends explicitly on filter-aaaa.@SO@ so it triggers the build even when that should not happen.

It is not enough to fix this by changing the install prerequisites to use ${TARGETS} because the install command still tries to install the missing filter-aaaa.@SO@ which fails.

I'm not sure what the best way to do conditional installs is, so I don't have a fix.