Only search the project source directory when URHO3D_HOME is defined.

Source/CMake/Modules/Urho3D-CMake-common.cmake

@@ -511,7 +511,10 @@ endmacro ()
 macro (add_android_native_init)
     # This source file could not be added when building SDL static library because it needs SDL_Main() which is not yet available at library building time
     # The SDL_Main() is defined by Android application that could be resided in other CMake projects outside of Urho3D CMake project which makes things a little bit complicated
-    find_file (ANDROID_MAIN_C_PATH SDL_android_main.c PATHS ${URHO3D_HOME}/Source/ThirdParty/SDL/src/main/android DOC "Path to SDL_android_main.c" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
+    if (URHO3D_HOME)
+        # First attempt to search using project source directory
+        find_file (ANDROID_MAIN_C_PATH SDL_android_main.c PATHS ${URHO3D_HOME}/Source/ThirdParty/SDL/src/main/android DOC "Path to SDL_android_main.c" NO_DEFAULT_PATH NO_CMAKE_FIND_ROOT_PATH)
+    endif ()
     if (NOT ANDROID_MAIN_C_PATH)
         # Attempt another search using Urho3D SDK installation location which could be rooted
         find_file (ANDROID_MAIN_C_PATH SDL_android_main.c PATH_SUFFIXES ${PATH_SUFFIX} DOC "Path to SDL_android_main.c")