Support a headless EGL option.

This is a new option to support completely headless running of Dolphin without X11 on systems that can properly support it.

Source/Core/DolphinWX/CMakeLists.txt

@@ -194,7 +194,7 @@ if(wxWidgets_FOUND)
 	set(CPACK_PACKAGE_EXECUTABLES ${CPACK_PACKAGE_EXECUTABLES} ${DOLPHIN_EXE})
 endif()
 
-if(USE_X11)
+if(USE_X11 OR ENABLE_HEADLESS)
 	set(DOLPHIN_NOGUI_EXE ${DOLPHIN_EXE_BASE}-nogui)
 	add_executable(${DOLPHIN_NOGUI_EXE} ${SRCS} ${NOGUI_SRCS})
 	target_link_libraries(${DOLPHIN_NOGUI_EXE} ${LIBS})