glsl: Fix crashes caused by Bison error messages involving "'%'".

Invalid shaders containing the character % at an unexpected location
would cause Bison to call yyerror with a message of:

    syntax error, unexpected '%'

Bison expects yyerror() to take a string, while _mesa_glsl_error() is a
printf-style function.  This hit the classic printf string escape issue:

    _mesa_glsl_error(loc, state, "unexpected '%'");       // invalid!
    _mesa_glsl_error(loc, state, "%s", "unexpected '%'"); // correct.

This caused assertion failures after ralloc_asprintf_append called
vsnprintf to determine the length of the text that would be printed:
vsnprintf would see the invalid format and return -1, an invalid length.

The solution is to define a proper yyerror() wrapper function that calls
_mesa_glsl_error with the "%s".  Since we compile with -p "_mesa_glsl",
yyerror is defined as:

    #define yyerror         _mesa_glsl_error

So we have to #undef yyerror in order to be able to declare it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43564
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Acked-by: Paul Berry <[email protected]>

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