autotools: use an absolute path to test data
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: Since test_bitcoin is being built and run inside src/test/, try using relative directories from that point
Merge pull request #2979 from luke-jr/autoconf

Autoconf fixes