Issue 1:
Using --target feature in rust will hard-code the provided path in the library. Linking these libraries against applications will fail if they use same target.json but with a different (e.g., relative) path.

I tried this code:

lib.rs:

#![feature(no_std)]
#![no_std]

main.rs

#![feature(lang_items, start, no_std)]
#![no_std]
extern crate lib;

#[lang="phantom_fn"]
trait PhantomFn<A: ?Sized, R: ?Sized = ()> {}
#[lang="sized"]
trait Sized: PhantomFn<Self> {}
#[lang="copy"]
trait Copy: PhantomFn<Self> {}
#[lang="sync"]
trait Sync: PhantomFn<Self> {}

#[start]
fn start() {
}
#[lang = "stack_exhausted"] extern fn stack_exhausted() {}
#[lang = "eh_personality"] extern fn eh_personality() {}
#[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }

Now, I compile the lib.rs as a library

rustc --crate-type=lib --emit=link -O --cfg arch__x86_64 --target=target.json --crate-name lib lib.rs

...and main.rs as an application using liblib.rlib (I'm in folder play):

Works:
rustc --emit=obj -L dependency=./ -O --cfg arch__x86_64 --target=target.json --extern lib=./liblib.rlib -o main.o main.rs
Does not work:
rustc --emit=obj -L dependency=./ -O --cfg arch__x86_64 --target=../play/target.json --extern lib=./liblib.rlib -o main.o main.rs

Instead, this happened:

rustc --emit=obj -L dependency=./ -O --cfg arch__x86_64 --target=../play/target.json --extern lib=./liblib.rlib -o main.o main.rs

main.rs:4:1: 4:18 error: couldn't find crate `lib` with expected target triple ../play/target.json
main.rs:4 extern crate lib;
^~~~~~~~~~~~~~~~~
main.rs:4:18: 4:18 note: crate `lib`, path #1, triple target.json: /home/gz/workspace/play/liblib.rlib
error: aborting due to previous error

Meta

rustc --version --verbose:

rustc 1.0.0-nightly (c89de2c56 2015-03-28) (built 2015-03-29)
binary: rustc
commit-hash: c89de2c56baeb61e7cc434924dcc8bedd32b26b8
commit-date: 2015-03-28
build-date: 2015-03-29
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly

Issue 2:
the ./configure output is :

configure: looking for configure programs

configure: found program cmp

configure: found program mkdir

configure: found program printf

configure: found program cut

configure: found program head

configure: found program grep

configure: found program xargs

configure: found program cp

configure: found program find

configure: found program uname

configure: found program date

configure: found program tr

configure: found program sed

configure: found program file

configure: found program make

configure: inspecting environment

configure: recreating config.tmp

configure:

configure: processing ./configure args

configure:

configure: CFG_LOCALSTATEDIR := /var/lib

configure: CFG_SYSCONFDIR := /etc

configure: CFG_DATADIR := /share

configure: CFG_INFODIR := /share/info

configure: CFG_LLVM_ROOT :=

configure: CFG_JEMALLOC_ROOT :=

configure: CFG_BUILD := x86_64-apple-darwin

configure: CFG_ANDROID_CROSS_PATH := /opt/ndk_standalone

configure: CFG_RELEASE_CHANNEL := dev

configure: CFG_MUSL_ROOT := /usr/local

configure: CFG_BUILD := x86_64-apple-darwin

configure: CFG_LIBDIR := /usr/local/lib

configure:

configure: validating ./configure args

configure:

configure: CFG_BOOTSTRAP_KEY := 21:11:31

configure:

configure: looking for build programs

configure:

configure: CFG_CURLORWGET := /usr/bin/curl (7.37.1)

configure: CFG_PYTHON := /usr/bin/python2.7

configure: CFG_GIT := /usr/bin/git (2.3.8)

configure: git: no git directory. disabling submodules

configure: CFG_MD5 := /sbin/md5

configure: CFG_MD5SUM :=

configure: CFG_HASH_COMMAND := /sbin/md5 -q | head -c 8

configure: CFG_CLANG := /usr/bin/clang++ (7.0.0)

configure: CFG_CCACHE :=

configure: CFG_GCC := /usr/bin/gcc (7.0.0)

configure: CFG_LD := /usr/bin/ld

configure: CFG_VALGRIND :=

configure: CFG_PERF :=

configure: CFG_ISCC :=

configure: CFG_ANTLR4 :=

configure: CFG_GRUN :=

configure: CFG_FLEX := /usr/bin/flex (2.5.35)

configure: CFG_BISON := /usr/bin/bison (2.3)

configure: CFG_PANDOC :=

configure: CFG_XELATEX :=

configure: CFG_GDB :=

configure: CFG_LLDB := /usr/bin/lldb (340.4.53.3)

configure: CFG_DISABLE_VALGRIND_RPASS := 1

configure: CFG_LLDB_VERSION := lldb-340.4.53.3

configure: CFG_LLDB_PYTHON_DIR := /Library/Developer/CommandLineTools ...

configure:

configure: looking for target specific programs

configure:

configure: CFG_ADB :=

configure:

configure: on OS X >=10.9, forcing use of clang

configure:

configure: CFG_ENABLE_CLANG := 1

configure: CFG_USING_CLANG := 1

configure: error: bad CLANG version: 7.0.0, need >=3.0svn