fix rvalue reference

[ariesdevil]

Fix compile error on alpine linux, the error is following:

pip install libmc
Collecting libmc
  Using cached libmc-1.1.0.tar.gz
Installing collected packages: libmc
  Running setup.py install for libmc ... error
    Complete output from command /usr/bin/python2 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-0reiZW/libmc/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-0brqPz-record/install-record.txt --single-version-externally-managed --compile:
    running install
    running build
    running build_py
    creating build
    creating build/lib.linux-x86_64-2.7
    creating build/lib.linux-x86_64-2.7/libmc
    copying libmc/__init__.py -> build/lib.linux-x86_64-2.7/libmc
    running build_ext
    cythoning libmc/_client.pyx to libmc/_client.cpp
    building 'libmc._client' extension
    creating build/temp.linux-x86_64-2.7
    creating build/temp.linux-x86_64-2.7/src
    creating build/temp.linux-x86_64-2.7/libmc
    gcc -fno-strict-aliasing -Os -fomit-frame-pointer -g -DNDEBUG -Os -fomit-frame-pointer -g -fPIC -Iinclude -I/usr/include/python2.7 -c src/BufferReader.cpp -o build/temp.linux-x86_64-2.7/src/BufferReader.o -fno-strict-aliasing -fno-exceptions -fno-rtti -Wall -DMC_USE_SMALL_VECTOR -O3 -DNDEBUG
    gcc -fno-strict-aliasing -Os -fomit-frame-pointer -g -DNDEBUG -Os -fomit-frame-pointer -g -fPIC -Iinclude -I/usr/include/python2.7 -c src/Connection.cpp -o build/temp.linux-x86_64-2.7/src/Connection.o -fno-strict-aliasing -fno-exceptions -fno-rtti -Wall -DMC_USE_SMALL_VECTOR -O3 -DNDEBUG
    In file included from src/Connection.cpp:6:0:
    /usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
     #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
      ^~~~~~~
    src/Connection.cpp: In member function 'ssize_t douban::mc::Connection::send()':
    src/Connection.cpp:234:76: error: invalid initialization of non-const reference of type 'size_t& {aka long unsigned int&}' from an rvalue of type 'size_t {aka long unsigned int}'
       msg.msg_iov = const_cast<struct iovec *>(m_buffer_writer->getReadPtr(msg.msg_iovlen));
                                                                            ~~~~^~~~~~~~~~
    In file included from include/Connection.h:11:0,
                     from src/Connection.cpp:13:
    include/BufferWriter.h:29:29: note:   initializing argument 1 of 'const iovec* const douban::mc::io::BufferWriter::getReadPtr(size_t&)'
       const struct iovec* const getReadPtr(size_t &n);
                                 ^~~~~~~~~~
    error: command 'gcc' failed with exit status 1

    ----------------------------------------
Command "/usr/bin/python2 -u -c "import setuptools, tokenize;__file__='/tmp/pip-build-0reiZW/libmc/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-0brqPz-record/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /tmp/pip-build-0reiZW/libmc/

This PR also check if glibc is exists.
@youngsofun @MOON-CLJ plz review

[youngsofun]

LGTM