Journald logging support was merged. This means developers now need to install journald headers:
EL7: yum install systemd-devel
Fedora: dnf install systemd-devel
Debian/Ubuntu: apt install libsystemd-dev
The alternative is to exclude the journald group: bundle install --without journald
https://github.com/theforeman/foreman/commit/abbe2a0494aa500c632702b9abea1357e7f573aa
And to reply on my own topic: on Fedora 27 I get an error with systemd-devel-234-10.git5f8984e.fc27.x86_64 installed
gcc -I. -I/usr/include -I/usr/include/ruby/backward -I/usr/include -I.
-DRUBY_EXTCONF_H=\"extconf.h\" -I/usr/include -fPIC -std=c11 -m64 -o
journald_native.o -c journald_native.c
In file included from journald_native.c:21:0:
sd_journal.h:40:14: error: #error Required function sd_journal_print is missing
#error Required function sd_journal_print is missing
^~~~~
sd_journal.h:44:14: error: #error Required function sd_journal_sendv is missing
#error Required function sd_journal_sendv is missing
^~~~~
sd_journal.h:48:14: error: #error Required function sd_journal_perror is missing
#error Required function sd_journal_perror is missing
^~~~~
make: *** [Makefile:243: journald_native.o] Error 1
I have the very same version and it was the only thing I had to do.