centos 6 CLOUDFLARE_ZLIB='y' workaround - when CLANG='y' set default clang 3.4.2 fails to compile nginx with CLOUDFLARE_ZLIB='y' set, so fall back to GCC 5.3.1 via devtoolset-4. However, this update sets fallback to GCC 6.3.1 via devtoolset-6 instead...