Fix ccache search in Android CMake toolchain.

android/android.toolchain.cmake

@@ -1031,6 +1031,9 @@ endif()
 # ccache support
 __INIT_VARIABLE( _ndk_ccache NDK_CCACHE ENV_NDK_CCACHE )
 if( _ndk_ccache )
+ if( DEFINED NDK_CCACHE AND NOT EXISTS NDK_CCACHE )
+  unset( NDK_CCACHE CACHE )
+ endif()
  find_program( NDK_CCACHE "${_ndk_ccache}" DOC "The path to ccache binary")
 else()
  unset( NDK_CCACHE CACHE )