Automatic variables, filename & syntax functions

[ihh]

Implements the following

• a comprehensive test suite (60 tests and counting)
• various ways of setting variables (recursive & simple)
• appending to variables
• multi-line variables
• more automatic variables such as $^ and $(@f) etc.
• substitution references
• computed variable names
• all the text functions
• all the filename functions
• the shell function
• user-defined functions
• errors and warnings
• various other quirks of GNU make syntax e.g. single-line recipes, forced rebuilds

Still to come: environment variables, override/undefine keywords, conditional blocks, conditional functions, iterators... 100% compatibility is not really feasible and some of these may be more important than others...