glsl/glcpp: Fixup glcpp tests for redefining a macro with whitespace changes.

glsl/glcpp: Fixup glcpp tests for redefining a macro with whitespace changes.

Previously, the test suite was expecting the compiler to allow a redefintion
of a macro with whitespace added, but gcc is more strict and allows only for
changes in the amounts of whitespace, (but insists that whitespace exist or
not in exactly the same places).

See: https://gcc.gnu.org/onlinedocs/cpp/Undefining-and-Redefining-Macros.html:

 These definitions are effectively the same:

      #define FOUR (2 + 2)
      #define FOUR         (2    +    2)
      #define FOUR (2 /* two */ + 2)

 but these are not:

      #define FOUR (2 + 2)
      #define FOUR ( 2+2 )
      #define FOUR (2 * 2)
      #define FOUR(score,and,seven,years,ago) (2 + 2)

This change adjusts the existing "redefine-macro-legitimate" test to work with
the more strict understanding, and adds a new "redefine-whitespace" test to
verify that changes in the position of whitespace are flagged as errors.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>