fix install path for cmake files

The installation path of FindXXX.cmake is absolute `/usr/cmake-2.8/Modules/` without respecting `${CMAKE_INSTALL_PREFIX}`. This leads to error if we want to make an installation with custom prefix and without root rights. 

This patch corrects this, by introducing a variable `INSTALL_CMAKE_DIR` with default of `lib/CMake/avcd` taken from the [cmake tutorial](http://www.cmake.org/Wiki/CMake/Tutorials/How_to_create_a_ProjectConfig.cmake_file#The_main_FooBar.2FCMakeLists.txt_file).

regards, 
  Alexander