Build issue on Mac OS X 10.9.5

Building with:

```
[zjb238:~/code/ps-lite]$ g++ -v (master)
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc/5.2.0/libexec/gcc/x86_64-apple-darwin13.4.0/5.2.0/lto-wrapper
Target: x86_64-apple-darwin13.4.0
Configured with: ../configure --build=x86_64-apple-darwin13.4.0 --prefix=/usr/local/Cellar/gcc/5.2.0 --libdir=/usr/local/Cellar/gcc/5.2.0/lib/gcc/5 --enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-5 --with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr --with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl --with-system-zlib --enable-libstdcxx-time=yes --enable-stage1-checking --enable-checking=release --enable-lto --with-build-config=bootstrap-debug --disable-werror --with-pkgversion='Homebrew gcc 5.2.0' --with-bugurl=https://github.com/Homebrew/homebrew/issues --enable-plugin --disable-nls --enable-multilib
Thread model: posix
gcc version 5.2.0 (Homebrew gcc 5.2.0)
```

I get the following error:

``````
asic_stringstream, std::__1::allocator > in libzmq.a(libzmq_la-address.o)
construction vtable for std::__1::basic_iostream >-in-std::__1::basic_stringstream, std::__1::allocator > in libzmq.a(libzmq_la-tcp_address.o)
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
make: *** [guide/example_a] Error 1```
``````