boost install with cxx14 support

by default boost has build with cxx11 support on mac 10.13 high sierra with xcode 9.4

to avoid link errors with c++14,it should build with c++14 support

./bootstrap.sh  --prefix=path

./b2 link=static cxxflags="-std=c++14" -d+2 stage

// ./b2 .....

//./b2 install

//on gentoo may has    error: "none" is not a known value of feature

//this because boost installed and the file /usr/share/boost-build/site-config.jam

//add this --ignore-site-config