Cleanup LevelDB library dependencies
configure: Internal changes to make building bitcoind and bitcoin-cli optional
configure: Change --with[out]-qt to --enable-frontends=bitcoind,bitcoin-qt to support more build configurations
configure: Minor grammatical changes to be more clear
Bugfix: Undefine _FORTIFY_SOURCE before redefining it, to avoid warnings on compilers that define it by default
Bugfix: Use BOOST_CPPFLAGS when testing for boost sleep vs sleep_for