meson: Fix winflexbison warnings

Undefine __STDC_VERSION__ for C files to avoid mismatch with C11/C17.

Define __STDC_VERSION__ for C++ files to use <inttypes.h> path.

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10196>

src/compiler/glsl/meson.build

@@ -44,7 +44,7 @@ glsl_lexer_cpp = custom_target(
   'glsl_lexer_cpp',
   input : 'glsl_lexer.ll',
   output : 'glsl_lexer.cpp',
-  command : [prog_flex, '-o', '@OUTPUT@', '@INPUT@'],
+  command : [prog_flex_cpp, '-o', '@OUTPUT@', '@INPUT@'],
 )
 
 ir_expression_operation_constant_h = custom_target(