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>

meson.build

@@ -1734,15 +1734,24 @@ if build_machine.system() == 'windows'
   if prog_flex.found()
     # windows compatibility (uses <io.h> instead of <unistd.h> and _isatty,
     # _fileno functions)
-    prog_flex = [prog_flex, '--wincompat', '-D__STDC_VERSION__=199901']
+    prog_flex = [prog_flex, '--wincompat']
+    if get_option('c_std') == 'c99'
+      prog_flex += '-D__STDC_VERSION__=199901'
+    endif
   else
     prog_flex = [find_program('flex', 'lex', required : with_any_opengl)]
   endif
   # Force flex to use const keyword in prototypes, as relies on __cplusplus or
   # __STDC__ macro to determine whether it's safe to use const keyword, but
-  # MSVC never defines __STDC__ unless we disable all MSVC extensions.
+  # MSVC only defines __STDC_VERSION__ for C11/C17.
   prog_flex += '-DYY_USE_CONST='
 
+  prog_flex_cpp = prog_flex
+  if get_option('c_std') != 'c99'
+    # Convince win_flex to use <inttypes.h> for C++ files
+    prog_flex_cpp += '-D__STDC_VERSION__=199901'
+  endif
+
   prog_bison = find_program('win_bison', required : false)
   if not prog_bison.found()
     prog_bison = find_program('bison', 'yacc', required : with_any_opengl)
@@ -1763,6 +1772,7 @@ else
   endif
 
   prog_flex = find_program('flex', required : with_any_opengl)
+  prog_flex_cpp = prog_flex
 endif
 
 dep_selinux = null_dep

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(