configure: Simplify common AM_CPPFLAGS and AM_LDFLAGS to a Makefile.common
configure: Check common include subdirectories for bdb headers, and refuse to use any version other than 4.8 by default
build-unix.md: Remove now-redundant and unnecessary Gentoo instructions
configure: Fix missing libbitcoin.a->libleveldb.a dependency, and add unsupported --with-system-leveldb configure flag
configure: Prefer specific known-good versions of Qt metacompiler tools
configure: Internal changes to make building bitcoind 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
configure: Change default CXXFLAGS to optimise debug builds slightly
configure: Disable debug by default (build release)
Bugfix: configure: Allow user-provided CXXFLAGS to override default optimisation, debug, and warning flags