Windows uv build is busted

[brson]

These 'overriding commands...' lines usually indicate a mistake in the makefiles

$ make VERBOSE=1
cfg: build triple i686-pc-mingw32
cfg: host triples i686-pc-mingw32
cfg: target triples i686-pc-mingw32
cfg: host for i686-pc-mingw32 is i386
cfg: os for i686-pc-mingw32 is pc-mingw32
cfg: using gcc
cfg: disabling valgrind due to its unreliability on this platform
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: overriding commands fo
r target `i686-pc-mingw32/stage0/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: ignoring old commands
for target `i686-pc-mingw32/stage0/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: overriding commands fo
r target `i686-pc-mingw32/stage1/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: ignoring old commands
for target `i686-pc-mingw32/stage1/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: overriding commands fo
r target `i686-pc-mingw32/stage2/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: ignoring old commands
for target `i686-pc-mingw32/stage2/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: overriding commands fo
r target `i686-pc-mingw32/stage3/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/target.mk:95: warning: ignoring old commands
for target `i686-pc-mingw32/stage3/bin/rustc/i686-pc-mingw32/bin/'
/c/rustbuild/scratch/rust-brson/mk/host.mk:138: warning: overriding commands for
 target `i686-pc-mingw32/stage1/bin/'
/c/rustbuild/scratch/rust-brson/mk/host.mk:138: warning: ignoring old commands f
or target `i686-pc-mingw32/stage1/bin/'
/c/rustbuild/scratch/rust-brson/mk/host.mk:138: warning: overriding commands for
 target `i686-pc-mingw32/stage2/bin/'
/c/rustbuild/scratch/rust-brson/mk/host.mk:138: warning: ignoring old commands f
or target `i686-pc-mingw32/stage2/bin/'
/c/rustbuild/scratch/rust-brson/mk/host.mk:138: warning: overriding commands for
 target `i686-pc-mingw32/stage3/bin/'
/c/rustbuild/scratch/rust-brson/mk/host.mk:138: warning: ignoring old commands f
or target `i686-pc-mingw32/stage3/bin/'
cfg: no pandoc found, omitting docs
cfg: no node found, omitting docs
cfg: no llnextgen found, omitting grammar-verification
cfg: no pandoc found, omitting library doc build

The build ends in failure

gcc -DRUST_NDEBUG -fno-omit-frame-pointer -O2 -I/home/Thad/rust/src/libuv/includ
e -I/home/Thad/rust/src/libuv/include/uv-private  -c -o /home/Thad/rust/src/libu
v/src/win/winapi.o /home/Thad/rust/src/libuv/src/win/winapi.c
gcc -DRUST_NDEBUG -fno-omit-frame-pointer -O2 -I/home/Thad/rust/src/libuv/includ
e -I/home/Thad/rust/src/libuv/include/uv-private  -c -o /home/Thad/rust/src/libu
v/src/win/winsock.o /home/Thad/rust/src/libuv/src/win/winsock.c
gcc -DRUST_NDEBUG -fno-omit-frame-pointer -O2 -c /home/Thad/rust/src/libuv/src/f
s-poll.c -o src/fs-poll.o
C:/MinGW/msys/1.0/home/Thad/rust/src/libuv/src/fs-poll.c:22:16: fatal error: uv.
h: No such file or directory
compilation terminated.
make[2]: *** [src/fs-poll.o] Error 1
make[2]: Leaving directory `/home/Thad/rust/rt/i686-w64-mingw32/libuv'
make[1]: *** [/home/Thad/rust//rt/i686-w64-mingw32/libuv] Error 2
make[1]: Leaving directory `/home/Thad/rust/src/libuv'
make: *** [rt/i686-w64-mingw32/libuv/libuv.a] Error 2

As reported by Thad. Not sure how the bots continue to run.

[yichoi]

#6235 cause duplicated rules for LIB and BIN in case of Windows (4dd0fa6#L1R86, 175a5ee#L0R133)

It can be simply fixed with CFG_LIB check whether bin or lib however considering multiple target triples with linux and windows, CFG_LIB needs to configurable #5223 and #5577