added verification of submodules for android

the current steps for android build didn't have new notion of needing to clone the submodules

android/README.md

@@ -20,6 +20,10 @@ Since Vulkan is not yet part of the Android OS (like OpenGL ES) the library and
 
 ### Building the Examples
 
+### Grab Externals
+
+Make sure all submodules have been cloned using the [steps from main README](../README.md#cloning).
+
 ### Complete set
 
 **Please note that building (and deploying) all examples may take a while**

android/build.py

@@ -71,6 +71,12 @@ for arg in sys.argv[1:]:
         BUILD_ARGS = "APP_CFLAGS=-D_VALIDATION"
         break
 
+# Verify submodules are loaded in external folder
+if not os.listdir("../external/glm/") or not os.listdir("../external/gli/"):
+    print("External submodules not loaded. Clone them using:")
+    print("\tgit submodule init\n\tgit submodule update")
+    sys.exit(-1)
+
 # Build
 os.chdir(PROJECT_FOLDER)