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>

test/example/Jamfile

@@ -15,6 +15,6 @@ alias fat-tests ;
 
 explicit fat-tests ;
 
-run $(TEST_MAIN) $(SOURCES) : : : : run-fat-tests ;
+alias run-fat-tests : common//run-tests ;
 
 explicit run-fat-tests ;