cmake: don't hard-code absolute path name /etc

- this breaks packaging and installation into a prefix in general
- there's a GNUInstallDirs variable for /etc, too

CMakeLists.txt

@@ -349,7 +349,7 @@ endif()
 
 # copy pam service to /etc/pam.d
 if(EXISTS "/etc/pam.d")
-    install(FILES "${CMAKE_SOURCE_DIR}/pam/netopeer2.conf" DESTINATION "/etc/pam.d")
+    install(FILES "${CMAKE_SOURCE_DIR}/pam/netopeer2.conf" DESTINATION "${CMAKE_INSTALL_SYSCONFDIR}/pam.d")
 endif()
 
 if(SYSREPO_SETUP)