Missing unistd header builderrors

Build stopped with errors, fixed on my system by including `<unistd.h>` header

```
c++ -v
Using built-in specs.
COLLECT_GCC=c++
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-amazon-linux/4.8.2/lto-wrapper
Target: x86_64-amazon-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --with-linker-hash-style=gnu --enable-languages=c,c++,fortran,ada,lto --enable-plugin --enable-initfini-array --disable-libgcj --without-isl --without-cloog --with-tune=generic --with-arch_32=i686 --build=x86_64-amazon-linux
Thread model: posix
gcc version 4.8.2 20131212 (Red Hat 4.8.2-7) (GCC) 
```

``` 
uname -a
Linux ip-10-0-0-189 3.10.40-50.136.amzn1.x86_64 #1 SMP Tue May 13 21:35:08 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux
```