fix XCode build (#304)

- Revives original solution proposed in #220
- Adds exclusion of GLFW library, XCode fails to link this

Author: svenevs

CMakeLists.txt

@@ -381,10 +381,30 @@ add_library(nanogui-obj OBJECT
   include/nanogui/nanogui.h
 )
 
-add_library(nanogui ${NANOGUI_LIBRARY_TYPE}
-  $<TARGET_OBJECTS:nanogui-obj>
-  $<TARGET_OBJECTS:glfw_objects>
-)
+# XCode has a serious bug where the XCode project produces an invalid target
+# that will not get linked if it consists only of objects from object libraries,
+# it will not generate any products (executables, libraries). The only work
+# around is to add a dummy source file to the library definition. This is an
+# XCode, not a CMake bug. See: https://itk.org/Bug/view.php?id=14044
+if (CMAKE_GENERATOR STREQUAL Xcode)
+  set(XCODE_DUMMY ${CMAKE_CURRENT_BINARY_DIR}/xcode_dummy.cpp)
+  file(WRITE ${XCODE_DUMMY} "")
+  add_library(nanogui ${NANOGUI_LIBRARY_TYPE}
+    ${XCODE_DUMMY}
+    $<TARGET_OBJECTS:nanogui-obj>
+    $<TARGET_OBJECTS:glfw_objects>
+  )
+
+  # XCode also fails at linking GLFW, so we exclude it.  Note that the actual
+  # component we need to build is 'glfw_objects', which is merged into the
+  # NanoGUI library itself.
+  set_target_properties(glfw PROPERTIES EXCLUDE_FROM_ALL 1 EXCLUDE_FROM_DEFAULT_BUILD 1)
+else()
+  add_library(nanogui ${NANOGUI_LIBRARY_TYPE}
+    $<TARGET_OBJECTS:nanogui-obj>
+    $<TARGET_OBJECTS:glfw_objects>
+  )
+endif()
 
 if (NANOGUI_BUILD_SHARED)
   set_property(TARGET nanogui-obj PROPERTY POSITION_INDEPENDENT_CODE ON)