What am I missing?
Trying to build on `Slackware 14.2 (x64)` box failed.

I had to patch `CMakeLists.txt` to accept the cmake version installed.

```diff
-if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.6.0") 
+if (${CMAKE_VERSION} VERSION_GREATER "3.5.1") 
```

And I ended up with this:

```sh
[ 61%] Building CXX object CMakeFiles/p2pool.dir/src/block_cache.cpp.o
In file included from /.../p2pool/src/common.h:282:0,
                 from /.../p2pool/src/block_cache.cpp:18:
/.../p2pool/src/util.h:125:13: error: specialization of ‘template<class _Tp> struct std::hash’ in different namespace [-fpermissive]
 struct std::hash<p2pool::hash>
             ^
In file included from /usr/include/c++/5.5.0/bits/basic_string.h:5574:0,
                 from /usr/include/c++/5.5.0/string:52,
                 from /usr/include/c++/5.5.0/stdexcept:39,
                 from /usr/include/c++/5.5.0/array:38,
                 from /usr/include/c++/5.5.0/tuple:39,
                 from /usr/include/c++/5.5.0/functional:55,
                 from /.../p2pool/src/common.h:41,
                 from /.../p2pool/src/block_cache.cpp:18:
/usr/include/c++/5.5.0/bits/functional_hash.h:58:12: error:   from definition of ‘template<class _Tp> struct std::hash’ [-fpermissive]
     struct hash;
            ^
CMakeFiles/p2pool.dir/build.make:182: recipe for target 'CMakeFiles/p2pool.dir/src/block_cache.cpp.o' failed
make[2]: *** [CMakeFiles/p2pool.dir/src/block_cache.cpp.o] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/p2pool.dir/all' failed
make[1]: *** [CMakeFiles/p2pool.dir/all] Error 2
Makefile:127: recipe for target 'all' failed
make: *** [all] Error 2
```