support for GNU configure syntax
Correct prefix / CFG_PREFIX work in configure / install.mk
support for GNU configure syntax corrections: patch from brson
Prepared `std::sys` for removal, and made `begin_unwind` simpler

- `begin_unwind` is now generic over any `T: Any + Send`.
- Every value you fail with gets boxed as an `~Any`.
- Because of implementation details, `&'static str` and `~str` are still
  handled specially behind the scenes.
- Changed the big macro source string in libsyntax to a raw string
  literal, and enabled doc comments there.
syntax: Forbid non-string literals in meta items. #623
Make Writer::flush a no-op default method

Closes #9126
auto merge of #10120 : Kimundi/rust/remove_sys, r=alexcrichton

- `begin_unwind` and `fail!` is now generic over any `T: Any + Send`.
- Every value you fail with gets boxed as an `~Any`.
- Because of implementation issues, `&'static str` and `~str` are still
  handled specially behind the scenes.
- Changed the big macro source string in libsyntax to a raw string
  literal, and enabled doc comments there.
Forbid type parameters in inner statics

Closes #9186
auto merge of #10164 : brson/rust/configure, r=brson

Rebase of #9990
auto merge of #10166 : brson/rust/meta, r=alexcrichton

This doesn't fix #623 but works around it by limiting the grammar.
auto merge of #10180 : alexcrichton/rust/flush-default, r=brson

Closes #9126
auto merge of #10189 : alexcrichton/rust/inner-statics, r=cmr

Closes #9186