set correct mandir

Index: cmake/FreecivInstall.cmake
--- cmake/FreecivInstall.cmake.orig
+++ cmake/FreecivInstall.cmake
@@ -185,7 +185,7 @@ if(UNIX AND NOT APPLE)
       install(
         FILES
         ${CMAKE_BINARY_DIR}/docs/man/freeciv21-client.6
-        DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/man/man6
+        DESTINATION ${CMAKE_INSTALL_PREFIX}/man/man6
         COMPONENT freeciv21
       )
     endif()
@@ -218,7 +218,7 @@ if(UNIX AND NOT APPLE)
         ${CMAKE_BINARY_DIR}/docs/man/freeciv21-game-manual.6
         ${CMAKE_BINARY_DIR}/docs/man/freeciv21-manual.6
         ${CMAKE_BINARY_DIR}/docs/man/freeciv21-ruleup.6
-        DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/man/man6
+        DESTINATION ${CMAKE_INSTALL_PREFIX}/man/man6
         COMPONENT freeciv21
       )
     endif()
@@ -249,7 +249,7 @@ if(UNIX AND NOT APPLE)
         FILES
         ${CMAKE_BINARY_DIR}/docs/man/freeciv21-modpack-qt.6
         ${CMAKE_BINARY_DIR}/docs/man/freeciv21-modpack.6
-        DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/man/man6
+        DESTINATION ${CMAKE_INSTALL_PREFIX}/man/man6
         COMPONENT freeciv21
       )
     endif()
