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/bench/parser/Jamfile

@@ -7,7 +7,7 @@
 # Official repository: https://github.com/boostorg/beast
 #
 
-run $(TEST_MAIN) nodejs_parser.cpp bench_parser.cpp : : : : bench-parser ;
+run  nodejs_parser.cpp bench_parser.cpp : : : : bench-parser ;
 
 explicit bench-parser ;