[linux]修复 gcc 编译时的出现的 "CPU you selected does not support x86-64 instruction set" 错误

$ pip install gevent
Downloading/unpacking gevent
  Running setup.py egg_info for package gevent
    ....

    building 'gevent.core' extension
    gcc -pthread -DNDEBUG -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -D_GNU_SOURCE -fPIC -fwrapv -O2 -mcpu=i686 -march=i686 -fPIC -DLIBEV_EMBED=1 -DEV_COMMON= -DEV_CLEANUP_ENABLE=0 -DEV_EMBED_ENABLE=0 -DEV_PERIODIC_ENABLE=0 -Ibuild/temp.linux-x86_64-2.6/libev -Ilibev -I/usr/include/python2.6 -c gevent/gevent.core.c -o build/temp.linux-x86_64-2.6/gevent/gevent.core.o

    gevent/gevent.core.c:1: error: CPU you selected does not support x86-64 instruction set

    error: command 'gcc' failed with exit status 1

    ....

$ CFLAGS='-march=x86-64'
$ pip install gevent