Document environment variables we handle, and don't mention -pipe (it isn't yet supported... lazy developers).
Document environment variables we handle, and don't mention -pipe (it
isn't yet supported... lazy developers).


git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@71109 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang