add find_bugs makefile target for undefined functions

[rnewson]

some false positives for hastings (part of geo feature not present in couchdb core) but the rest need fixin'

[rnewson]

current output on main

 make find_bugs | grep Warn | grep -v hastin
Warning: couch_config:delete/2 is undefined function (Xref)
Warning: couch_config:get/2 is undefined function (Xref)
Warning: couch_config:parse_ini_file/1 is undefined function (Xref)
Warning: couch_config:set/3 is undefined function (Xref)
src/couch_plugins.erl:121: Warning: couch_plugins:delete_config/1 calls undefined function couch_config:delete/2 (Xref)
src/couch_plugins.erl:112: Warning: couch_plugins:load_config_file/2 calls undefined function couch_config:parse_ini_file/1 (Xref)
src/couch_plugins.erl:19: Warning: couch_plugins:plugin_dir/0 calls undefined function couch_config:get/2 (Xref)
src/couch_plugins.erl:83: Warning: couch_plugins:register_plugin/2 calls undefined function couch_config:set/3 (Xref)
src/couch_plugins.erl:117: Warning: couch_plugins:set_config/1 calls undefined function couch_config:set/3 (Xref)
src/couch_plugins.erl:87: Warning: couch_plugins:unregister_plugin/1 calls undefined function couch_config:delete/2 (Xref)
src/couch_plugins_httpd.erl:18: Warning: couch_plugins_httpd:handle_req/1 calls undefined function couch_config:get/2 (Xref)
src/couch_plugins_httpd.erl:53: Warning: couch_plugins_httpd:plugin_dir/0 calls undefined function couch_config:get/2 (Xref)
file.erl: Warning: file:delete/2 is undefined function (Xref)
src/smoosh_persist.erl:136: Warning: smoosh_persist:delete_file/1 calls undefined function file:delete/2 (Xref)
src/ioq.erl: Warning: ioq:get_disk_queues/0 is undefined function (Xref)
src/weatherreport_check_ioq.erl:88: Warning: weatherreport_check_ioq:check_legacy/1 calls undefined function ioq:get_disk_queues/0 (Xref)

[rnewson]

the smoosh_persist one is because it uses a function only present in OTP 24 and I tested on OTP 23 which is the lowest OTP couchdb officially supports (and thus it needs fixing).