cmake: GNUInstallDirs for Debian compatibility

On Debian, the packaging helpers and infrastructure rely on cmake
projects including this CMake module -- otherwise various features such
as cross-compiling won't work correctly.

Author: jktjkt

CMakeLists.txt

@@ -1,6 +1,8 @@
 cmake_minimum_required(VERSION 2.8.9)
 project(libyang C)
 
+include(GNUInstallDirs)
+
 set(LIBYANG_DESCRIPTION "libyang is YANG data modelling language parser and toolkit written (and providing API) in C.")
 
 # set version