rules: CC/auto: add "config_file" rule to generate a c/c++ header...

...starting from a template (aka configuration file), and using the
variables defined via a ["CC/auto", "config"] target.

For example, to use a CMake configuration file, the targets could be
defined as follows

	...
	, "foo-header-blueprint":
	  { "type": ["@", "rules", "CC/auto", "config_file"]
	  , "input": ["config.hpp.in"]
	  , "output": ["config.hpp"]
	  , "stage": ["foo"]
	  , "magic_string": ["cmakedefine"]
	  , "@only": ["true"]
	  }
	, "foo-header":
	  { "type": "configure"
	  , "target": "foo-header-blueprint"
	  , "config":
	    { "type": "let*"
	    , "bindings":
	      [ [ "defines"
	        , [ ["var", "\"string value\""]
	          , ["FOO_MAJOR_VERSION", "3"]
	          , ["use_this_feature", true]
	          ]
	        ]
	      ]
	    , "body": {"type": "env", "vars": ["defines"]}
	    }
	  }
	  ...

The file config.hpp.in may look as follows

	#ifndef config_cmake
	#define config_cmake

	#cmakedefine var
	#cmakedefine use_this_feature
	#cmakedefine01 use_this_feature
	#cmakedefine unused

	#define FOO_VERSION @FOO_MAJOR_VERSION@
	#define DONT_TOUCH_THIS ${FOO_MAJOR_VERSION}

	#endif

and the generated configuration file foo/config.hpp is

	#ifndef config_cmake
	#define config_cmake

	#define var "string value"
	#define use_this_feature
	#define use_this_feature 1
	/* #undef unused */

	#define FOO_VERSION 3
	#define DONT_TOUCH_THIS ${FOO_MAJOR_VERSION}

	#endif

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