Don't generate undefined variable warnings for variables that are
internal / special to make and where the empty string is valid.
Rather than defining them to empty, which could introduce unwanted
behavior, keep a list of variable names which we should never warn
about.
* src/variable.h (warn_undefined): Convert the macro to a function.
* src/variable.c (defined_vars): Always "defined" variable names.
(warn_undefined): Implement as a function and check against the
defined_vars before generating a warning.
* src/read.c (read_all_makefiles): No need to reset warning flag.
* src/vpath.c (build_vpath_lists): Ditto.
* tests/scripts/options/warn-undefined-variables: Expand all the
pre-defined variables to ensure warnings are not generated.
