Rust conf_test

Run configuration tests from build.rs and set available features, similar to autotools configure scripts.

Description

ConfTest::run() called from 'build.rs' parses 'Cargo.toml'. Then for each [feature] defined, it checks if that feature was not set manually (with --features) and a test in 'conf_tests/' exists. This test is then compiled and build. When that succeeds the feature becomes enabled automatically.