Fixed compilation errors for Windows multi-DLL build.

third_party/mesa/MesaLib/include/GL/gl.h

 /*
  * Mesa 3-D graphics library
  * Version:  7.6
  *
  * Copyright (C) 1999-2006  Brian Paul   All Rights Reserved.
  * Copyright (C) 2009  VMware, Inc.  All Rights Reserved.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
  * to deal in the Software without restriction, including without limitation
@@ skipping 37 matching lines @@
 #endif
 
 #if defined(_WIN32) && !defined(__WIN32__) && !defined(__CYGWIN__)
 #define __WIN32__
 #endif
 
 #if !defined(OPENSTEP) && (defined(__WIN32__) && !defined(__CYGWIN__))
 #  if (defined(_MSC_VER) || defined(__MINGW32__)) && defined(BUILD_GL32) /* tag specify we're building mesa as a DLL */
 #    define GLAPI __declspec(dllexport)
 #  elif (defined(_MSC_VER) || defined(__MINGW32__)) && defined(_DLL) /* tag specifying we're building for DLL runtime support */
-#    define GLAPI __declspec(dllimport)
+// We always retrieve the entry points dynamically via GetProcAddress or
+// OSMesaGetProcAddress. This works around an issue where using the MSVC
+// multi-threaded runtime library, which defines _DLL.
+//#    define GLAPI __declspec(dllimport)
+#    define GLAPI
 #  else /* for use with static link lib build of Win32 edition only */
 #    define GLAPI extern
 #  endif /* _STATIC_MESA support */
 #  if defined(__MINGW32__) && defined(GL_NO_STDCALL) || defined(UNDER_CE)  /* The generated DLLs by MingW with STDCALL are not compatible with the ones done by Microsoft's compilers */
 #    define GLAPIENTRY 
 #  else
 #    define GLAPIENTRY __stdcall
 #  endif
 #elif defined(__CYGWIN__) && defined(USE_OPENGL32) /* use native windows opengl32 */
 #  define GLAPI extern
@@ skipping 2148 matching lines @@
 /*
  * End system-specific stuff
  **********************************************************************/
 
 
 #ifdef __cplusplus
 }
 #endif
 
 #endif /* __gl_h_ */