Add ALLOW_MISSING_GCONF build option

CMakeLists.txt

@@ -15,6 +15,7 @@ add_custom_target(uninstall
 )
 
 option(ENABLE_GCONF_MIGRATION "Enables GConf to GSettings migration for existing alarms (and adds a dependency to GConf)." ON)
+option(ALLOW_MISSING_GCONF "Allows the project to build with GConf missing. Useful for existing installations (AUR) that already had GConf." OFF)
 if(ENABLE_GCONF_MIGRATION)
     add_subdirectory("gconf-migration")
 endif()

gconf-migration/CMakeLists.txt

@@ -1,6 +1,14 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
-pkg_check_modules(GCONF2 REQUIRED gconf-2.0)
 pkg_check_modules(GIO REQUIRED gio-2.0)
+pkg_check_modules(GCONF2 gconf-2.0)
+if(NOT GCONF2_FOUND)
+    if(ALLOW_MISSING_GCONF)
+        message(WARNING "GConf was not found but the package will still be built.\nIf you want to migrate old alarms created by alarm-clock-applet < 0.4.0, please install GConf and re-run CMake/rebuild the package.")
+        return()
+    else()
+        message(FATAL_ERROR "GConf not found.\nPlease install it or pass `-DENABLE_GCONF_MIGRATION=OFF` if you do not want to migrate alarms created by alarm-clock-applet < 0.4.0.")
+    endif()
+endif()
 
 add_executable(alarm-clock-applet-gconf-migration alarm-clock-gconf-migration.c)
 set_property(TARGET alarm-clock-applet-gconf-migration PROPERTY C_STANDARD 11)