pkg-config: make prefix overridable again

While we don't support prefix being != /usr, and this is hardcoded
all over the place, variables in pkg-config file are expected
to have overridable base directory.

This is important for at least the following two use cases:

- Installing projects to non-FHS package-specific prefixes for Nix-style
  package managers. Of course, it is then their responsibility
  to ensure systemd can find the service files.
- Installing to local path for development purposes.
  This is a compromise between running a program from a build directory,
  and running it fully installed to system prefix.

You will not want to write to system prefix in either case.

For more information, see also
https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/

Fixes https://github.com/systemd/systemd/issues/18082

Partially reverts 6e65df89c3

src/core/systemd.pc.in

@@ -26,10 +26,10 @@ systemdsystemunitdir=${systemd_system_unit_dir}
 systemd_system_preset_dir=${rootprefix}/lib/systemd/system-preset
 systemdsystempresetdir=${systemd_system_preset_dir}
 
-systemd_user_unit_dir=/usr/lib/systemd/user
+systemd_user_unit_dir=${prefix}/lib/systemd/user
 systemduserunitdir=${systemd_user_unit_dir}
 
-systemd_user_preset_dir=/usr/lib/systemd/user-preset
+systemd_user_preset_dir=${prefix}/lib/systemd/user-preset
 systemduserpresetdir=${systemd_user_preset_dir}
 
 systemd_system_conf_dir=${sysconfdir}/systemd/system
@@ -47,7 +47,7 @@ systemduserunitpath=${systemd_user_unit_path}
 systemd_system_generator_dir=${root_prefix}/lib/systemd/system-generators
 systemdsystemgeneratordir=${systemd_system_generator_dir}
 
-systemd_user_generator_dir=/usr/lib/systemd/user-generators
+systemd_user_generator_dir=${prefix}/lib/systemd/user-generators
 systemdusergeneratordir=${systemd_user_generator_dir}
 
 systemd_system_generator_path=/run/systemd/system-generators:/etc/systemd/system-generators:/usr/local/lib/systemd/system-generators:${systemd_system_generator_dir}
@@ -62,7 +62,7 @@ systemdsleepdir=${systemd_sleep_dir}
 systemd_shutdown_dir=${root_prefix}/lib/systemd/system-shutdown
 systemdshutdowndir=${systemd_shutdown_dir}
 
-tmpfiles_dir=/usr/lib/tmpfiles.d
+tmpfiles_dir=${prefix}/lib/tmpfiles.d
 tmpfilesdir=${tmpfiles_dir}
 
 sysusers_dir=${rootprefix}/lib/sysusers.d
@@ -77,7 +77,7 @@ binfmtdir=${binfmt_dir}
 modules_load_dir=${rootprefix}/lib/modules-load.d
 modulesloaddir=${modules_load_dir}
 
-catalog_dir=/usr/lib/systemd/catalog
+catalog_dir=${prefix}/lib/systemd/catalog
 catalogdir=${catalog_dir}
 
 system_uid_max=@SYSTEM_UID_MAX@