Improve installation for non-depends builds

[ryanofsky]

bitcoin/bitcoin#16367 (review) reported various problems getting multiprocess code to build outside the make depends system. I've only focused on make depends builds so far, but would like to create libmultiprocess packages and add better instructions to make installation in other contexts easier.

[ryanofsky]

@ariard can you post more details here? Main thing I'd like to know is how you installed capnp (via package manager or with manual install) and what platform or distro you used to make sure I can reproduce the problems.

If you have still have the build commands you used or any error messages / build output, it'd be great to post that as well. Thanks!

[ryanofsky]

Talked to @ariard, turns out he was doing a depends build of capnproto and libmultiprocess, but a non-depends build of bitcoin itself, due to not passing --prefix=$PWD/depends/x86_64-pc-linux-gnu to the bitcoin configure script. Passing the --prefix flag should fix all the issues encountered. To prevent this confusion in the future I think the depends README needs to be updated to clearly state that depends outputs will be ignored if prefix is not specified.

The other thing that should be be done is to update the build-osx and build-unix files to mention how to install capnproto and libmultiprocess as optional dependencies, maybe just linking to instructions at the bottom of the libmultiprocess README

[hebasto]

#4 (comment):

... would like to create libmultiprocess packages...

See #96.