Rename LIBDIR_RELATIVE to CFG_LIBDIR_RELATIVE

[brson]

This looks bad:

configure: CFG_SRC_DIR          := /home/brian/rust-0.9/
configure: CFG_BUILD_DIR        := /home/brian/rust-0.9/
configure: CFG_OSTYPE           := unknown-linux-gnu
configure: CFG_CPUTYPE          := x86_64
configure: CFG_CONFIGURE_ARGS   := --prefix=/home/brian/rust-0.9-insta ...
configure: CFG_PREFIX           := /home/brian/rust-0.9-install
configure: CFG_BUILD            := x86_64-unknown-linux-gnu
configure: CFG_HOST             := x86_64-unknown-linux-gnu
configure: CFG_TARGET           := x86_64-unknown-linux-gnu
configure: CFG_C_COMPILER       := gcc
configure: CFG_LIBDIR           := /home/brian/rust-0.9-install/lib
configure: CFG_RUSTLIBDIR       := rustlib
configure: LIBDIR_RELATIVE      := lib
configure: CFG_DISABLE_MANAGE_SUBMODULES := 1
configure: CFG_ANDROID_CROSS_PATH := /opt/ndk_standalone
configure: CFG_MINGW32_CROSS_PATH :=
configure: CFG_MANDIR           := /home/brian/rust-0.9-install/share/ ...
configure: CFG_LLVM_ROOT        :=
configure: CFG_LLVM_SRC_DIR     := /home/brian/rust-0.9/src/llvm/
configure: CFG_LLVM_BUILD_DIR_x86_64_unknown_linux_gnu := /home/brian/rust-0.9/x86_64-unknown ...
configure: CFG_LLVM_INST_DIR_x86_64_unknown_linux_gnu := /home/brian/rust-0.9/x86_64-unknown ...

[jhasse]

I've named it LIBDIR_RELATIVE because it isn't configurable (you can't change it using ./configure --libdir-relative=foo). Maybe it shouldn't be printed at all? After all CFG_LIBDIR already tells enough.