Why pcb-rnd plugins are in /usr/lib/ and not /usr/lib64/
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On a 64 bit system where both 32 bit and 64 bit software needs to be run
a separate set of 32 and 64 bit libraries are kept for two reasons:

1. You may want to install both the 32 and the 64 bit version of the
same lib (e.g. libc) without interference.

2. You want the dynamic linker to find the right one for your app; so
when linking a 32 bit app, you want to look only for 32 bit libs and
when linking a 64 bit app, you want to look only for 64 bit libs.
Linking means the dynamic linker's automatic run when a dynamic linked
executable is started up.

What we do in our plugin dir is neither of those. We never want to have
both 32 and 64 bit pcb-rnds live side by side on a 64 bit system. And
these .so files are never read by the dynamic linker on executable
startup. The only one way they are loaded is by explicit path, using
dlopen, from within pcb-rnd. Because they are plugins, not general
purpose shared libs. So it is safe to put them in a common lib/, we do
not need to install them in different libdirs per system.