GNU make-style variables

[ihh]

Brings variables into line with GNU make in various ways, including:

• $A for single-letter variable names, $(ABC) for multi-letter variable names
• allow variables to be specified from the plmake command line with ABC=foo syntax
• three different ways to set a variable, as per here:
  • ABC := $(DEF) defines a simply expanded variable, which means $(DEF) is evaluated at the time of assignment. This was the former default
  • ABC = $(DEF) defines a recursively expanded variable, which just means $(DEF) is evaluated whenever $(ABC) is used (I think of this as lazy evaluation, rather than being particularly recursive; hopefully I have not misunderstood what GNU make is doing here)
  • ABC ?= $(DEF) is a conditional variable assignment, so variable ABC is only overwritten if it has not already been set (e.g. on the command line)
  • NB there are new operators for := and ?= so you can also use them in the Prolog dialect of makespec.pro makeprogs

Also added

• change in default makefile-seeking behavior (now looks for a Prolog makespec.pro, then a GNU Makefile)
• include directives for GNU Makefiles, so you can recursively include makefiles inside other makefiles, as per here
• line numbers for syntax errors when parsing GNU Makefiles, which work in include'd files
• some examples in examples/Makefile.foo (probably should make these into proper tests... any thoughts on a good test framework?)

Prolog is kicking my arse in general, I am still only half-understanding most of the code, but this all seems to work... famous last words.