Allow C++ header files in header compiler
Add SRCDIR directive

This currently needs to be passed at the very start of the Configfile,
which isn't actually strictly necessary.  I'm going to improve this in
a bit...
Allow SRCDIR anywhere

It doesn't make a whole lot of sense to just stick SRCDIR anywhere,
but there are at least a few places (ie, right under a BINARIES) that
it does make sense.

This is pretty liberal about allowing it places where you can break
your Configfile, but I guess that's life!
Bump version to 0.7.0