Added suppport for C_EXTERN.

Allows include directories to be specified that will be ingnored
even if they are included with double-quotes.
Moved over to local copy of the new std.process.

This was a bit tricky because all the linux-specific stuff like
chaning commands, redirection and setting environent variables on the
command-line don't work with the new process module.

This change is a step towards making bob work under windows and OSX.

The big remaining change re portability is to make the signal handling
code portable.

Also, there is a bug in the signal handling area re shutting down the
reently introduced thread that handles the signals. The whole area needs
rewriting.
New method for specifying external libraries is working

This is a significant improvement on the old approach, and greatly
simplifies maintenance of Bobfiles. It is particularly useful on
projects that reuse in-house components as locally installed binary
packages raher than building everything from source.
Assorted bug-fixes, mostly reverting to linux-specific process spawning
Workaround "temporary can't be an lvalue/ref" issue.
Build locally
Added a .gitignore