glsl/glcpp: Fix preprocessor error condition for macro redefinition

glsl/glcpp: Fix preprocessor error condition for macro redefinition

This patch specifically fixes redefinition condition for white space
changes. #define and #undef functionality in GLSL follows the standard
for C++ preprocessors for macro definitions.

From 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)

Fixes Khronos GLES3 CTS tests;
invalid_object_whitespace_vertex
invalid_object_whitespace_fragment

Signed-off-by: Anuj Phogat <anuj.phogat@gmail.com>
Reviewed-by: Carl Worth <cworth@cworth.org>