rustc: Disable rpath settings by default

This commit disables rustc's emission of rpath attributes into dynamic libraries
and executables by default. The functionality is still preserved, but it must
now be manually enabled via a `-C rpath` flag.

This involved a few changes to the local build system:

* --disable-rpath is now the default configure option
* Makefiles now prefer our own LD_LIBRARY_PATH over the user's LD_LIBRARY_PATH
  in order to support building rust with rust already installed.
* The compiletest program was taught to correctly pass through the aux dir as a
  component of LD_LIBRARY_PATH in more situations.

The major impact of this change is that neither rustdoc nor rustc will work
out-of-the-box in all situations because they are dynamically linked. It must be
arranged to ensure that the libraries of a rust installation are part of the
LD_LIBRARY_PATH. The default installation paths for all platforms ensure this,
but if an installation is in a nonstandard location, then configuration may be
necessary.

Additionally, for all developers of rustc, it will no longer be possible to run
$target/stageN/bin/rustc out-of-the-box. The old behavior can be regained
through the `--enable-rpath` option to the configure script.

This change brings linux/mac installations in line with windows installations
where rpath is not possible.

Closes #11747
[breaking-change]
add "(see below)" to a reference to a new concept

This is because I observed someone reading the tutorial who thought they'd
missed something when they got to the mention of variable bindings.

This patch doesn't reflow the paragraphs so that you can see the semantic
change that I made, and a subsequent patch will reflow this paragraph.

Fixes https://github.com/rust-lang/rust/issues/13570.
reflow with emacs fill-paragraph (fill-column 71)
Register new snapshots

This change starts denying `*T` in the parser. All code using `*T` should ensure
that the FFI call does indeed take `const T*` on the other side before renaming
the type to `*const T`.

Otherwise, all code can rename `*T` to `*const T`.

[breaking-change]
Guide: Hello, cargo
Add a note that this requires `git`.
Fix capitalization of Issue.

@chris-morgan and @huonw pointed out that even though the feature
is a proper noun, we're using it in a more generic sense here.
tha -> than

Thanks @huonw :heart:
all/some

How could I forsake `rustdoc`? :sweat_smile:

Thanks @huonw.
staticly -> statically

What's funny about this one is that spellcheck caught it, but for
some reason didn't give me the right suggestion, so I assumed that it
wasn't in my dictionary. Oh well.

Thanks @P1start! :heart:
Fix IRC linkage.

Death to `here`!

Thanks @chris-morgan :heart:
StackOverflow -> Stack Overflow

Thanks @chris-morgan
Fixing link to subreddit.

Thanks @chris-morgan :heart:
Move note about $ up

Thanks @chris-morgan :heart:
TIL PATHEXT

Thanks @chris-morgan :heart:
Clarify which curly is being referred to

Thanks @chris-morgan
Note that macros != templates

Good call, @chris-morgan and @cmr!
Fix missing word and some grammar

Thanks @chris-morgan and @P1start! :heart:
C macros, not C++ templates.

@cmr rightfully points out that C macros are worse, and share the
same name.
README: Add instructions for building on windows.