Macro assistance part 1 - macro directive

Macro assistance part 1 - macro directive

Define the &:macro directive, which is a dollar-doubled version of
make's own `define'.

We must introuce a new concept of `nesting', for when "the content of
the construct is $-doubled" in the words of the README.  The
implementation is in the main process_input_mk function: it is a
multi-line scope of some kind with an ad-hoc ending condition, and its
own dollar-doubling setting.

We will also want a convenient syntax for $(eval $(call...)), which
we're going to introduce in a moment.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>