Resolved compiler errors related to disabling SDL. Now we've just got a ton of linker errors to deal with...

engine/Quake/gl_vidsdl.c

@@ -22,6 +22,8 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 */
 // gl_vidsdl.c -- SDL GL vid component
 
+#if defined(USE_LIBSDL) && defined(USE_OPENGL)
+
 #include "quakedef.h"
 #include "cfgfile.h"
 #include "bgmusic.h"
@@ -2333,3 +2335,4 @@ static void VID_Menu_f (void)
 	VID_Menu_RebuildRateList ();
 }
 
+#endif	// USE_LIBSDL && USE_OPENGL

engine/Quake/in_sdl.c

@@ -21,6 +21,8 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 
 */
 
+#ifdef USE_LIBSDL
+
 #include "quakedef.h"
 #if defined(SDL_FRAMEWORK) || defined(NO_SDL_CONFIG)
 #if defined(USE_SDL2)
@@ -1138,3 +1140,4 @@ void IN_SendKeyEvents (void)
 	}
 }
 
+#endif	// USE_LIBSDL

engine/Quake/pl_win.c

@@ -21,6 +21,8 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 
 */
 
+#ifdef USE_LIBSDL
+
 #include "quakedef.h"
 #include <windows.h>
 #if defined(SDL_FRAMEWORK) || defined(NO_SDL_CONFIG)
@@ -112,3 +114,4 @@ void PL_ErrorDialog(const char *errorMsg)
 			MB_OK | MB_SETFOREGROUND | MB_ICONSTOP);
 }
 
+#endif	// USE_LIBSDL

engine/Quake/sys_sdl_win.c

@@ -21,6 +21,8 @@ Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 
 */
 
+#ifdef USE_LIBSDL
+
 #ifndef WIN32_LEAN_AND_MEAN
 #define WIN32_LEAN_AND_MEAN
 #endif
@@ -441,3 +443,4 @@ void Sys_SendKeyEvents (void)
 	IN_SendKeyEvents();
 }
 
+#endif	// USE_LIBSDL

engine/UniQuake/sys_uniquake.c

@@ -98,18 +98,40 @@ void Sys_mkdir(const char *path)
 		Sys_Error("Unable to create directory %s", path);
 }
 
-void Sys_Sleep_New(unsigned long msecs)
+const char *Sys_ConsoleInput(void)
+{
+	// Not implemented: we support neither STDIN nor dedicated servers in UniQuake
+	return NULL;
+}
+
+#ifndef USE_LIBSDL
+
+void Sys_Sleep(unsigned long msecs)
 {
 	// TODO: implement
 }
 
-void Sys_SendKeyEvents_New(void)
+void Sys_SendKeyEvents(void)
 {
 	// TODO: signal Unity to process input & send commands
 }
 
-const char *Sys_ConsoleInput(void)
+void PL_SetWindowIcon(void)
+{
+}
+
+void PL_VID_Shutdown(void)
+{
+}
+
+char *PL_GetClipboardData(void)
 {
-	// Not implemented: we support neither STDIN nor dedicated servers in UniQuake
 	return NULL;
 }
+
+void PL_ErrorDialog(const char *text)
+{
+	// TODO: could signal this back to Unity and show a platform-specific system dialog
+}
+
+#endif	// !USE_LIBSDL