zc.recipe.cmmi: Environment variables should understand shell syntax

[Thomas Lotze]

When specifying additional environment variables in a zc.recipe.cmmi buildout section, the values should be understood in a more shell-like way:

- variable names denoted by a $ sign should be expanded using the existing environment

- whitespace inside a value should be possible, protected by either quotes, single quotes or backslash escaping

These would allow for adding a new path to $PATH or specifying more than one compiler flag in $CFLAGS,