Build error on CentOS

@johnzachary reports a build error on CentOS:

```
vagrant-centos-64:~/git/taconic on feature/copse
$ buzzy build
[1] Clone git://github.com/redjack/copse.git (develop)
[2] Build copse 0.0+29+git (cmake)
make failed

stdout:
Scanning dependencies of target libcopse
[ 14%] Building C object src/CMakeFiles/libcopse.dir/libcopse/cps.c.o
[ 28%] Building C object src/CMakeFiles/libcopse.dir/libcopse/fiber.c.o
[ 42%] Building C object src/CMakeFiles/libcopse.dir/libcopse/round-robin.c.o
[ 57%] Building ASM object src/CMakeFiles/libcopse.dir/libcopse/context/jump_x86_64_sysv_elf_gas.S.o

stderr:
/usr/bin/as: unrecognized option '-error'
make[2]: *** [src/CMakeFiles/libcopse.dir/libcopse/context/jump_x86_64_sysv_elf_gas.S.o] Error 1
make[1]: *** [src/CMakeFiles/libcopse.dir/all] Error 2
make: *** [all] Error 2
```