gcc.jam: build and use import libraries on Cygwin and Windows, but
accept DLL-only linking with prebuilt DLLs.
builtin.jam: fix default-host-os so that <target-os> actually becomes
a functional feature. Improve English in comments
property-set.jam: add str method so we can print them in generator
debugging output.
generators.jam: improved debugging output
build-system.jam: add missing semicolon