Testing with mpost compiled from trunk

I'd like to try out the latest version of mpost which contains a couple of fixes to bugs I've reported.

Can someone point me at a simple set of instructions for trying the new mpost executable without clobbering my existing TeXLive distribution?

I have successfully cloned "trunk" into a local directory using anonymous svn and the build.sh ran without error and produced a new mpost exectable in ./build/texk/web2c, and if I run "./build/texk/web2c/mpost -v" I get

>  MetaPost 1.999 (TeX Live 2015) (kpathsea version 6.2.1)
>  etc...
>  etc...

as expected.

But if I try to run "./build/texk/web2c/mpost test.mp"  I get

> warning: kpathsea: configuration file texmf.cnf not found in these directories: [... long list of directories under . ]
> Sorry, I can't find the 'mpost' preload file; will try 'plain'.
> 
> I can't find the 'plain' preload file!

and it stops.  So I guess I need to build a preload file or point it at one.  

Are the instructions documented somewhere?

Thanks Toby
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http://tug.org/metapost/