Build busted on mac and older linux kernals

[jimfulton]

The current buildout relies on being able to treat scripts as executables. This only works on newer linux kernals.

The need for this seems to be driven by:

• The desire not to use git submodules to get persistent include files needed to build, and
• The weird way setup-requires handles dependencies.

Some alternatives:

• Use submodules. This is similar to the way we used to use svn externals.
• (re-) name the persistent and zope.interface eggs in the test scripts and use bake in a
  -s argument in the test runner to only run the BTrees tests.
• Try to fix buildout so that it forces the inclusion of persistent and zope.interface
  dependencies. I'm not entirely why this isn't happening now.

I vote for using submodules.

[tseaver]

We've used submodules to handle common theming for Pyramid / Pylons projects, and found them incredibly painful: ripping them out has been on the TODO list for a good while.

Note that I don't use the buildout, because I want to run tests using 'tox' (to insure that they pass on all supported Python versions).

[jimfulton]

We're using sub-modules on a few projects. We haven't found them
to be all that painful.

OK, then I'll study how we're using tox so as to not break it when I fix the
buildout. I'll probably just use the second option.