Add a cfg_attr syntax extension

This extends cfg-gating to attributes.

```rust
 #[cfg_attr(<cfg pattern>, <attr>)]
```
will expand to
```rust
 #[<attr>]
```
if the `<cfg pattern>` matches the current cfg environment, and nothing
if it does not. The grammar for the cfg pattern has a simple
recursive structure:

 * `value` and `key = "value"` are cfg patterns,
 * `not(<cfg pattern>)` is a cfg pattern and matches if `<cfg pattern>`
    does not.
 * `all(<cfg pattern>, ...)` is a cfg pattern and matches if all of the
    `<cfg pattern>`s do.
 * `any(<cfg pattern>, ...)` is a cfg pattern and matches if any of the
    `<cfg pattern>`s do.

Examples:

```rust
 // only derive Show for assert_eq! in tests
 #[cfg_attr(test, deriving(Show))]
 struct Foo { ... }

 // only derive Show for assert_eq! in tests and debug builds
 #[cfg_attr(any(test, not(ndebug)), deriving(Show))]
 struct Foo { ... }

 // ignore a test in certain cases
 #[test]
 #[cfg_attr(all(not(target_os = "linux"), target_endian = "big"), ignore)]
 fn test_broken_thing() { ... }

 // Avoid duplication when fixing staging issues in rustc
 #[cfg_attr(not(stage0), lang="iter")]
 pub trait Iterator<T> { ... }
```
Deprecate `#[ignore(cfg(...))]`

Replace `#[ignore(cfg(a, b))]` with `#[cfg_attr(all(a, b), ignore)]`
Fix rebase fallout
Correct typo in the Iterator adapters section
guide-pointers.md: C sample code should match the Rust version.

Fix rust-lang/rust#17255
Fix typo in unsafe guide (s/xlibrary/library)
Fix free lifetime vars in HashMap's iterators
Relicense shootout-fasta-redux.rs to the shootout license.

Everyone agreed.

Fix #17078
mk: Fix a minor makefile warning on windows
rustdoc: replace DIV inside H1 with SPAN.

W3C HTML5 spec states that H1 must enclose "phrasing content" which
doesn't include DIV.  But SPAN is OK.
http://www.w3.org/TR/html5/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
Fix typo, structs => fields.
debuginfo: Make sure that all calls to drop glue are associated with debug locations.

This commit makes rustc emit debug locations for all call
and invoke statements in LLVM IR, if they are contained
within a function that debuginfo is enabled for. This is
important because LLVM does not handle the case where a
function body containing debuginfo is inlined into another
function with debuginfo, but the inlined call statement
does not have a debug location. In this case, LLVM will
not know where (in terms of source code coordinates) the
function was inlined to and we end up with some statements
still linked to the source locations in there original,
non-inlined function without any indication that they are
indeed an inline-copy. Later, when generating DWARF from
the IR, LLVM will interpret this as corrupt IR and abort.

Unfortunately, the undesirable case described above can
still occur when using LTO. If there is a crate compiled
without debuginfo calling into a crate compiled with
debuginfo, we again end up with the conditions triggering
the error. This is why some LTO tests still fail with the
dreaded assertion, if the standard library was built with
debuginfo enabled.
That is, `RUSTFLAGS_STAGE2=-g make rustc-stage2` will
succeed but `RUSTFLAGS_STAGE2=-g make check` will still
fail after this commit has been merged. This is a problem
that has to be dealt with separately.

Fixes #17201
Fixes #15816
Fixes #15156
Correct tense
Mention array sugar in guide
configure: Fix the compatibility code for --enable-nightly

When --enable-nightly is not specified this expression is malformed
because $CFG_ENABLE_NIGHTLY is undefined.
Add Option::{ok_or, ok_or_else}

These are the inverses of `Result::ok` and help to bridge `Option` and
`Result` based APIs.
auto merge of #17506 : sfackler/rust/cfg-attr, r=alexcrichton

cc #17490 

Reopening of #16230
auto merge of #17469 : sfackler/rust/into-result, r=aturon

This is the inverse of `Result::ok` and helps to bridge `Option` and
`Result` based APIs.
auto merge of #17512 : nodakai/rust/fix-ptr-guide, r=alexcrichton

Fix rust-lang/rust#17255

This is such a trivial change.  Devs: perhaps you might want to omit to run `make check` on Travis.
auto merge of #17515 : mbrubeck/rust/patch-1, r=alexcrichton

r? @steveklabnik