Futureproof firefox rpath example

[jasom]

After a firefox upgrade, merely patching libxul.so was insufficient to
get apulse working on my machine.

In addition I use $LIB instead of lib so that the example can be used
unchanged for 32-bit applications on 64-bit hosts.

Also, since it uses find and file, it could theoretically be used to
patch an entire /usr tree safely. Whether or not to recommend this in
the readme I leave up to future changes.

[i-rinat]

That part of README file is just an example, not a full working solution. And I'd like to keep it that way. Changing it to a larger expression encourages to copy-paste it. But despite effort of making it compatible ($LIB special value), it probably won't work for many installations. For example in Debian/Ubuntu, apulse libraries could be in triplet directories, like /usr/lib/x86_64-linux-gnu/apulse.

I also did some tests regarding RPATH. Actually, it should work if firefox binary is modified. And it kind of does. I can see in traces that the directory is probed when firefox loads other libraries. But at some point it just forgets about additional path. I think, this is somehow related to libmozsandbox.so. In trace, it starts a new namespace, which probably throws away all additional paths. Setting RPATH on libxul.so works because it's loaded after.

In tests I performed just now, setting RPATH to libxul.so is still enough for official build of Firefox 53.0.3.

[i-rinat]

to patch an entire /usr tree

This makes no sense, as it's just equivalent of making apulse libraries accessible to any application. In that case, it's easier to just put them to /usr/lib or /usr/local/lib.

[i-rinat]

During tests I also found another way to teach Firefox about apulse. It is sufficient to copy (or make symlinks) libpulse.so.0 and libpulse-simple.so.0 to Firefox binary directory. firefox searches for libraries in its own directory too.