Make Metal an optional part of the build

See the corresponding commit in the main repo

QuartzCore/CAMetalDrawable.mm

@@ -23,6 +23,9 @@
 #import "CAMetalLayerInternal.h"
 #import <QuartzCore/CAMetalLayer.h>
 #import <Metal/MTLDeviceInternal.h>
+#import <Metal/stubs.h>
+
+#if DARLING_METAL_ENABLED
 
 #include <OpenGL/gl.h>
 #include <OpenGL/glext.h>
@@ -825,3 +828,18 @@ static void glDebugCallback(GLenum source, GLenum type, GLuint id, GLenum severi
 }
 
 @end
+
+#else
+
+@implementation CAMetalDrawableInternal
+
+@dynamic texture;
+@dynamic layer;
+@dynamic drawableID;
+@dynamic presentedTime;
+
+MTL_UNSUPPORTED_CLASS
+
+@end
+
+#endif

QuartzCore/CAMetalDrawableInternal.h

@@ -23,6 +23,8 @@
 #import <OpenGL/gl.h>
 #import <OpenGL/OpenGL.h>
 
+#if DARLING_METAL_ENABLED
+
 #include <indium/indium.private.hpp>
 
 @class CAMetalLayerInternal;
@@ -102,8 +104,12 @@ public:
 	void release();
 	void reset();
 };
+#endif
 
-@interface CAMetalDrawableInternal : NSObject <CAMetalDrawable, MTLDrawableInternal> {
+@interface CAMetalDrawableInternal : NSObject <CAMetalDrawable, MTLDrawableInternal>
+
+#if DARLING_METAL_ENABLED
+{
 	NSUInteger _drawableID;
 	CFTimeInterval _presentedTime;
 	std::shared_ptr<CAMetalDrawableActual> _drawable;
@@ -114,5 +120,6 @@ public:
 
 - (instancetype)initWithLayer: (CAMetalLayer*)layer
                      drawable: (std::shared_ptr<CAMetalDrawableActual>)drawable;
+#endif
 
 @end

QuartzCore/CAMetalLayer.mm

@@ -23,6 +23,7 @@
 #import <Foundation/NSRaise.h>
 #import <Metal/MTLDeviceInternal.h>
 #import <QuartzCore/CALayerContext.h>
+#import <Metal/stubs.h>
 
 #include <algorithm>
 
@@ -40,6 +41,7 @@ static void reportGLErrors(void) {
 
 @implementation CAMetalLayer
 
+#if DARLING_METAL_ENABLED
 // FIXME: this breaks inheritance from CAMetalLayer.
 //        the problem is that we need some C++ ivars, but we can't put those in the public header
 //        and this code needs to compile on 32-bit (so it can't use non-fragile ivars).
@@ -55,6 +57,7 @@ static void reportGLErrors(void) {
 		return [super allocWithZone: zone];
 	}
 }
+#endif
 
 - (id<MTLDevice>)device
 {
@@ -204,6 +207,8 @@ static void reportGLErrors(void) {
 
 @implementation CAMetalLayerInternal
 
+#if DARLING_METAL_ENABLED
+
 @synthesize presentsWithTransaction = _presentsWithTransaction;
 @synthesize displaySyncEnabled = _displaySyncEnabled;
 @synthesize wantsExtendedDynamicRangeContent = _wantsExtendedDynamicRangeContent;
@@ -590,4 +595,10 @@ static void reportGLErrors(void) {
 	}
 }
 
+#else
+
+MTL_UNSUPPORTED_CLASS
+
+#endif
+
 @end

QuartzCore/CMakeLists.txt

@@ -75,6 +75,12 @@ generate_sdk_framework(QuartzCore
     HEADER "include/QuartzCore"
 )
 
+if (BUILD_METAL)
+	set(QUARTZCORE_LINK_INDIUM_PRIVATE indium_private)
+else()
+	set(QUARTZCORE_LINK_INDIUM_PRIVATE "")
+endif()
+
 add_framework(QuartzCore
 	FAT
 	CURRENT_VERSION
@@ -90,7 +96,7 @@ add_framework(QuartzCore
 		OpenGL
 		CoreGraphics
 		Metal_private
-		indium_private
+		${QUARTZCORE_LINK_INDIUM_PRIVATE}
 		cxx
 	LINK_FLAGS
 	" -Wl,-reexport_library,${CMAKE_BINARY_DIR}/src/frameworks/CoreImage/CoreImage \