cmake: Use GNUInstallDirs to install data directories

CMakeLists.txt

@@ -3,6 +3,8 @@ cmake_minimum_required(VERSION 3.1.0)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
+include(GNUInstallDirs)
+
 # for link time optimization, remove if cmake version is >= 3.9
 if(POLICY CMP0069) # LTO
   cmake_policy(SET CMP0069 NEW)
@@ -186,7 +188,7 @@ elseif(UNIX)
     # Paths
     SET(BINDIR "${CMAKE_INSTALL_PREFIX}/bin" CACHE PATH "Where to install binaries")
     SET(LIBDIR "${CMAKE_INSTALL_PREFIX}/lib${LIB_SUFFIX}" CACHE PATH "Where to install libraries")
-    SET(DATAROOTDIR "${CMAKE_INSTALL_PREFIX}/share" CACHE PATH "Sets the root of data directories to a non-default location")
+    SET(DATAROOTDIR "${CMAKE_INSTALL_DATAROOTDIR}" CACHE PATH "Sets the root of data directories to a non-default location")
     SET(GLOBAL_DATA_PATH "${DATAROOTDIR}/games/" CACHE PATH "Set data path prefix")
     SET(DATADIR "${GLOBAL_DATA_PATH}/openmw" CACHE PATH "Sets the openmw data directories to a non-default location")
     SET(ICONDIR "${DATAROOTDIR}/pixmaps" CACHE PATH "Set icon dir")