Support GCC and clang PCH use and multiple or chaining PCH

[dmoody256]

SCons currently support use of a single PCH file on msvc builds.

However it is possible to chain PCH files so that a given compile job will use a PCH which actually represent multple PCH files.

Also add support with GCC and clang toolchains. I created a POC tool which removed the common PCH code from msvc.py and mslink.py and put them to use in a tool which can be used with several compilers.

This branch contains the changes to scons and the tool:
https://github.com/dmoody256/mongo/tree/pch_working

Pch tool file:
https://github.com/dmoody256/mongo/blob/pch_working/site_scons/site_tools/pch.py

[mwichmann]

See also #1435

[mwichmann]

See also #2489

[mwichmann]

These are no longer available?