build: Merge events and threads into single platform abstraction

build: Merge events and threads into single platform abstraction

The split between events and threads abstractions is unnecessary.
Simplify the library by merging the two into a "platform" abstraction.
The only meaningful change is that Cygwin builds will no longer use the
POSIX threads abstraction but will instead use the native Windows one.

The downside to doing this is that the dpfp_threaded example program
will no longer be available on Cygwin builds. This should be fine, and
future work will make dpfp_threaded available for all forms of Windows
build systems.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>