This was not a problem in 2.1.15, but is now a problem in 2.1.16.
Workaround:
ENV.append "LDFLAGS", "-lintl"
The error is
Undefined symbols for architecture x86_64: "_libintl_bind_textdomain_codeset", referenced from: _i18n_switchto_utf8 in libcommon.a(libcommon_a-i18n.o) _i18n_switchback in libcommon.a(libcommon_a-i18n.o) "_libintl_bindtextdomain", referenced from: _i18n_init in libcommon.a(libcommon_a-i18n.o) _i18n_localegettext in libcommon.a(libcommon_a-i18n.o) "_libintl_gettext", referenced from: _main in gpg_wks_client-gpg-wks-client.o _wrong_args in gpg_wks_client-gpg-wks-client.o _connect_dirmngr in gpg_wks_client-call-dirmngr.o __init_common_subsystems in libcommon.a(libcommon_a-init.o) _disable_core_dumps in libcommon.a(libcommon_a-sysutils.o) _gnupg_rename_file in libcommon.a(libcommon_a-sysutils.o) _i18n_utf8 in libcommon.a(libcommon_a-i18n.o) ... "_libintl_setlocale", referenced from: _i18n_init in libcommon.a(libcommon_a-i18n.o) _i18n_localegettext in libcommon.a(libcommon_a-i18n.o) _send_pinentry_environment in libcommon.a(libcommon_a-asshelp.o) "_libintl_textdomain", referenced from: _i18n_init in libcommon.a(libcommon_a-i18n.o)
Jenkins log:
https://bot.brew.sh/job/Homebrew%20Versions%20Pull%20Requests/1731/version=el_capitan/consoleFull