cmake: allow using system-installed readerwriterqueue

[alebcay]

readerwriterqueue can be installed into system directories, and a number of distributions already package it. Where possible, use the system-provided copy rather than requiring it to be downloaded via FetchContent.

Currently the default behavior provided by this PR is to use the system-provided copy if available; if preferred, this can also be inverted to prefer the FetchContent and only rely on the system version via an explicit option.

The objective of this PR is to allow downstream packagers to rely on their own system-installed copies of dependencies where possible rather than have them be downloaded at compile time. (I'd like to do the same for ni-midi2, but the build system for ni-midi2 does not yet facilitate installing it into system directories.)

If this change is not accepted, downstream packagers can still point FetchContent to a system-provided copy of readerwriterqueue, e.g. -DFETCHCONTENT_SOURCE_DIR_READERWRITERQUEUE:PATH=/usr/lib/cmake/readerwriterqueue.