Fix Blender crash on startup on macOS

The issue was caused by recent C++ transition: the header file is
shared between CPU and GPU. The Metal defines __cplusplus so it is
not enough to check for it to use C linking as it is not a valid
syntax for shaders.

Pull Request: https://projects.blender.org/blender/blender/pulls/110570

source/blender/draw/intern/draw_shader_shared.h

@@ -47,7 +47,7 @@ typedef enum eObjectInfoFlag eObjectInfoFlag;
 #  endif
 #endif
 
-#ifdef __cplusplus
+#if defined(__cplusplus) && !defined(GPU_SHADER)
 extern "C" {
 #endif
 
@@ -385,6 +385,6 @@ BLI_STATIC_ASSERT_ALIGN(DRWDebugPrintBuffer, 16)
 
 /** \} */
 
-#ifdef __cplusplus
+#if defined(__cplusplus) && !defined(GPU_SHADER)
 }
 #endif