Add ANDROID_GDBSERVER variable

Help/variable/ANDROID_GDBSERVER.rst

+ANDROID_GDBSERVER
+-----------------
+
+When :ref:`Cross Compiling for Android with the NDK`, this variable holds
+the absolute path to the ``gdbserver``.

Modules/Platform/Android-Initialize.cmake

 #=============================================================================
 # Copyright 2015-2016 Kitware, Inc.
+# Copyright 2016 Ruslan Baratov
 #
 # Distributed under the OSI-approved BSD License (the "License");
 # see accompanying file Copyright.txt for details.
@@ -49,3 +50,16 @@ else()
 endif()
 
 set(CMAKE_BUILD_TYPE_INIT Debug)
+
+if(CMAKE_ANDROID_NDK)
+  set(ANDROID_GDBSERVER
+    "${CMAKE_ANDROID_NDK}/prebuilt/android-${CMAKE_ANDROID_ARCH}/gdbserver/gdbserver"
+    CACHE
+    FILEPATH
+    "Path to gdbserver"
+    )
+
+  if(NOT EXISTS "${ANDROID_GDBSERVER}")
+    message(WARNING "gdbserver not found: ${ANDROID_GDBSERVER}")
+  endif()
+endif()