Building Ceph master with C++17 support on openSUSE Leap 42.3

Ceph now requires C++17 support, which is available with modern compilers such as gcc-7. openSUSE Leap 42.3, my current OS of choice, includes gcc-7. However, it's not used by default.

Using gcc-7 for the Ceph build is a simple matter of:

> sudo zypper in gcc7-c++
> CC=gcc-7 CXX=/usr/bin/g++-7 ./do_cmake.sh ...
> cd build && make -j$(nproc)