Add support for the POSIX :::= assignment operator.

POSIX Issue 8 will require a new assignment operator, :::=.
This operator behaves similarly to the BSD make := operator: the
right-hand side is expanded immediately, but then the value is
re-escaped (all '$' are converted to '$$') and the resulting variable
is considered a recursive variable: the value is re-expanded on use.

* src/variable.h (enum variable_flavor): Add f_expand flavor.
* src/variable.c (do_variable_definition): When defining f_expand,
post-process the result to re-escape '$' characters.
Remove default: to the compiler warns about un-handled enum values.
Set recursive values for both f_recursive and f_expand.
(parse_variable_definition): Rewrite this method.
The previous version was annoying to extend to ':::='.
(print_variable): Remove default: so the compiler warns us about
un-handled enum values.
* src/function.c (func_origin): Remove default: so the compiler warns
us about un-handled enum values.
* doc/make.texi: Add documentation for :::=.
* tests/scripts/variables/define: Add a test for define :::=.
* tests/scripts/variables/flavors: Add tests for :::=.
* tests/scripts/variables/negative: Add tests for :::=.

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