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/advanced/server-flex/Jamfile

@@ -11,7 +11,7 @@ import ac ;
 
 project
     : requirements
-        [ ac.check-library /boost/beast//static_ssl_asio : <library>/boost/beast//static_ssl_asio : <build>no ]
+        [ ac.check-library /boost/beast//static_ssl_asio : <library>/boost/beast//static_ssl_asio/<link>static : <build>no ]
     ;
 
 exe advanced-server-flex :

example/advanced/server/Jamfile

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