Fix broken Gradle Sync when opening the project with Android Studio

Summary:
This fixes an issue that I got reported by users attempting to open the react-native GitHub project in Android Studio.

The error is:
```
  Cannot specify include directories for target "react_codegen_AppSpecs" which is  not built by this project.
```

Changelog:
[Internal] [Changed] - Fix broken Gradle Sync when opening the project with Android Studio

Reviewed By: huntie

Differential Revision: D49189331

packages/rn-tester/NativeCxxModuleExample/CMakeLists.txt

@@ -20,10 +20,17 @@ file(GLOB nativecxxmoduleexample_SRC CONFIGURE_DEPENDS *.cpp)
 add_library(nativecxxmoduleexample STATIC ${nativecxxmoduleexample_SRC})
 
 target_include_directories(nativecxxmoduleexample PUBLIC .)
-target_include_directories(react_codegen_AppSpecs PUBLIC .)
 
 target_link_libraries(nativecxxmoduleexample
  fbjni
  jsi
- react_nativemodule_core
- react_codegen_AppSpecs)
+ react_nativemodule_core)
+
+# For Gradle Syncs (i.e. when you first open the project in Android Studio),
+# the react_codegen_AppSpecs target is not existing yet as it's generated by
+# Codegen. Therefore skip the linking with that library if it's missing in the
+# CMake dependency Graph. It will be included anyway in the final build.
+if (TARGET react_codegen_AppSpecs)
+ target_include_directories(react_codegen_AppSpecs PUBLIC .)
+ target_link_libraries(nativecxxmoduleexample react_codegen_AppSpecs)
+endif ()