external: don't define WIN32_LEAN_AND_MEAN just for shits and giggles.

We're not including windows.h there (fortunately!), so there's no point
in defining such a macro. Also the proper way would be defining it only
if it's not already defined to avoid macro redefinition warnings.

src/MagnumExternal/OpenGL/GL/flextGL.h

@@ -80,7 +80,6 @@ void flextGLInit(Magnum::GL::Context& context);
 #endif
 
 #if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__)
-#define WIN32_LEAN_AND_MEAN 1
 #ifndef WINAPI
 #define WINAPI __stdcall
 #endif

src/MagnumExternal/OpenGL/GL/flextGL.h.template

@@ -81,7 +81,6 @@ void flextGLInit(Magnum::GL::Context& context);
 #endif
 
 #if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__)
-#define WIN32_LEAN_AND_MEAN 1
 #ifndef WINAPI
 #define WINAPI __stdcall
 #endif

src/MagnumExternal/OpenGL/GLES2/flextGL.h

@@ -67,7 +67,6 @@ void flextGLInit(Magnum::GL::Context& context);
 #endif
 
 #if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__)
-#define WIN32_LEAN_AND_MEAN 1
 #ifndef WINAPI
 #define WINAPI __stdcall
 #endif

src/MagnumExternal/OpenGL/GLES2/flextGL.h.template

@@ -68,7 +68,6 @@ void flextGLInit(Magnum::GL::Context& context);
 #endif
 
 #if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__)
-#define WIN32_LEAN_AND_MEAN 1
 #ifndef WINAPI
 #define WINAPI __stdcall
 #endif

src/MagnumExternal/OpenGL/GLES3/flextGL.h

@@ -72,7 +72,6 @@ void flextGLInit(Magnum::GL::Context& context);
 #endif
 
 #if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__)
-#define WIN32_LEAN_AND_MEAN 1
 #ifndef WINAPI
 #define WINAPI __stdcall
 #endif

src/MagnumExternal/OpenGL/GLES3/flextGL.h.template

@@ -73,7 +73,6 @@ void flextGLInit(Magnum::GL::Context& context);
 #endif
 
 #if defined(_WIN32) && !defined(APIENTRY) && !defined(__CYGWIN__) && !defined(__SCITECH_SNAP__)
-#define WIN32_LEAN_AND_MEAN 1
 #ifndef WINAPI
 #define WINAPI __stdcall
 #endif