Fix failure of make mac64 due to version mismatch of cmake

[iwasakims]

make mac64 fails as reported in #257 . The cause is version mismatch of cmake.

$ make mac64
...
cd target/snappy-1.1.7-Mac-x86_64 && cmake  ../../target/snappy-1.1.7
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  CMake 3.1 or higher is required.  You are running version 3.0.2

Using latest multiarch/crossbuild based on Debian stretch should fix this.

[xerial]

Awesome! Thanks for the fix. I'll try this fix