support for GNU configure syntax
Correct prefix / CFG_PREFIX work in configure / install.mk
support for GNU configure syntax corrections: patch from brson
syntax: Forbid non-string literals in meta items. #623
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.