build: fix systemd-journal-upload installation

Fixes:

$ ./configure ... --disable-microhttpd --enable-libcurl
--enable-sysusers
$ make && make install DESTDIR=$(pwd)/INST
$ ls INST/usr/lib/sysusers.d/
basic.conf  systemd.conf

There is no a file with `systemd-journald-upload`

sysusers.d/.gitignore

@@ -1,2 +1,3 @@
 /basic.conf
 /systemd.conf
+/systemd-remote.conf

sysusers.d/systemd-remote.conf.m4

@@ -5,6 +5,10 @@
 #  the Free Software Foundation; either version 2.1 of the License, or
 #  (at your option) any later version.
 
+m4_ifdef(`HAVE_MICROHTTPD',
 u systemd-journal-gateway - "systemd Journal Gateway"
 u systemd-journal-remote  - "systemd Journal Remote"
+)m4_dnl
+m4_ifdef(`HAVE_LIBCURL',
 u systemd-journal-upload  - "systemd Journal Upload"
+)m4_dnl