build: Handle LLVM versions with a micro component

Configuring against LLVM 3.4 stable previously failed because
`llvm-config --version` returned 3.4.2.
scripts: Handle LLVM micro versions in tartan script

As per the previous commit. Also improve error handling when the
expected version is not supported, to make it more obvious what the
user can do to fix the situation.
scripts: Fix the path used when running tartan uninstalled

When calling the tartan script uninstalled, the $clang_bin_dir will be
the directory containing the tartan script — the scripts/ directory.
website: Fix running instructions to use `scan-build make`

Even if ./configure hard-codes the Clang compiler path into the
generated Makefiles, other environment variables it sets are needed at
compile time, so make must always be run inside scan-build. Otherwise
the analyser gets run without the Tartan plugin loaded, which is sad.
clang-plugin: UNFINISHED work to add reference count checking