Multiple OS support

Build scripts for Phobos, Proxygen, and Deimos work on CentOS7 and
CentOS8.

Change log:
* generalise build scripts for different OSs
* modularise CentOS7 specific building
* update Phobos to current master
* functions for building on CentOS8
* reflect new Phobos version
* link to more boost libraries for CentOS7 Proxygen build
1 job for 2-support-centos8