On systems that support both C compiling and dynamic loading, we can
speed up `realpath()` (where most time in rbenv is spent) by replacing
it with a dynamically loaded bash builtin.
When `make -C src` is called in the project's root,
`libexec/rbenv-realpath.dylib` will be created. If it exists, rbenv will
attempt to load it as a builtin command. If it fails, execution will
fall back to the old `realpath()` shell function.
readlink comes from GNU coreutils. On systems without it, rbenv used to
spin out of control when it didn't have readlink or greadlink available
because it would re-exec the frontend script over and over instead of the
worker script in libexec.
Fixes#389
