Bugfix: configure: TEST_DATA_DIR must be absolute, or it kinda defeats the purpose (to allow running from other directories)
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: Prefer specific known-good versions of Qt metacompiler tools
Bugfix: configure: Allow user-provided CXXFLAGS to override default optimisation, debug, and warning flags