Jamfile cleanup:

- Don't set the global link settings (`<link>`) to avoid dependency
ordering issues in the future.
- Allow the user to decide on the link settings for dependencies
(filesystem/coroutine).
- Make dependencies in examples explicit.
- Remove dependency of examples on Boost.Filesystem (was not used there).
- Deduplicate build settings.
- Speed up test compilation by avoiding rebuilding of main test file.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

example/websocket/server/async/Jamfile

@@ -12,4 +12,5 @@ exe websocket-server-async :
     :
     <variant>coverage:<build>no
     <variant>ubasan:<build>no
+    <library>/boost/beast//static_beast/<link>static
     ;