pkgconfig: define variables relative to ${prefix}/${rootprefix}/${sysconfdir}

pkgconfig: define variables relative to ${prefix}/${rootprefix}/${sysconfdir}

Fixes #4549.

People want to be able to redefine the prefixes relative to which the other
variables are defined. Something like
  pkgconf --define-variable=prefix=/home/user/installpath --variable=systemdsystemunitdir systemd

I'm not convinced that this entirely useful, because the installed systemd will
not look at those paths, but maybe it's OK as an alternative type of $DESTDIR.
This has been requested a few times over the years, so let's just provide this.

I thought this would be more complicated, since we allow all kinds of directories
to be overrides in the compilation configuration. But it turns out that all the
directories defined in systemd.pc are relative to three prefixes:
$prefix, $rootprefix, and $sysconfdir. So this patch adds $rootprefix and $sysconfdir
to the .pc file and then changes the subsequent definitions in the .pc file to use
them. In the end we define each path twice using the same rules: once in meson.build
and once in the .pc file.

Without overrides:
$ for i in $(pkgconf --with-path=build/src/core systemd --print-variables); do
     echo -n "$i = "; pkgconf --with-path=$PWD/build/src/core --variable=$i systemd
  done
containeruidbasemax = 1878982656
containeruidbasemin = 524288
dynamicuidmax = 65519
dynamicuidmin = 61184
systemgidmax = 999
systemuidmax = 999
catalogdir = /usr/lib/systemd/catalog
modulesloaddir = /usr/lib/modules-load.d
binfmtdir = /usr/lib/binfmt.d
sysctldir = /usr/lib/sysctl.d
sysusersdir = /usr/lib/sysusers.d
tmpfilesdir = /usr/lib/tmpfiles.d
systemdshutdowndir = /usr/lib/systemd/system-shutdown
systemdsleepdir = /usr/lib/systemd/system-sleep
systemdusergeneratordir = /usr/lib/systemd/user-generators
systemdsystemgeneratordir = /usr/lib/systemd/system-generators
systemduserunitpath = /etc/systemd/user:/etc/systemd/user:/run/systemd/user:/usr/local/lib/systemd/user:/usr/local/share/systemd/user:/usr/lib/systemd/user:/usr/lib/systemd/user:/usr/share/systemd/user
systemdsystemunitpath = /etc/systemd/system:/etc/systemd/system:/run/systemd/system:/usr/local/lib/systemd/system:/usr/lib/systemd/system:/usr/lib/systemd/system:/lib/systemd/system
systemduserconfdir = /etc/systemd/user
systemdsystemconfdir = /etc/systemd/system
systemduserpresetdir = /usr/lib/systemd/user-preset
systemduserunitdir = /usr/lib/systemd/user
systemdsystempresetdir = /usr/lib/systemd/system-preset
systemdsystemunitdir = /usr/lib/systemd/system
systemdutildir = /usr/lib/systemd
sysconfdir = /etc
rootprefix = /usr
prefix = /usr
pcfiledir = /usr/share/pkgconfig

With overrides:
$ for i in $(pkgconf --with-path=build/src/core systemd --print-variables); do
     echo -n "$i = "; pkgconf --with-path=$PWD/build/src/core \
     --define-variable=prefix=/PREFIX \
     --define-variable=rootprefix=/ROOTPREFIX \
     --define-variable=sysconfdir=/SYSCONF --variable=$i systemd
  done
containeruidbasemax = 1878982656
containeruidbasemin = 524288
dynamicuidmax = 65519
dynamicuidmin = 61184
systemgidmax = 999
systemuidmax = 999
catalogdir = /PREFIX/lib/systemd/catalog
modulesloaddir = /PREFIX/lib/modules-load.d
binfmtdir = /PREFIX/lib/binfmt.d
sysctldir = /PREFIX/lib/sysctl.d
sysusersdir = /PREFIX/lib/sysusers.d
tmpfilesdir = /PREFIX/lib/tmpfiles.d
systemdshutdowndir = /ROOTPREFIX/lib/systemd/system-shutdown
systemdsleepdir = /ROOTPREFIX/lib/systemd/system-sleep
systemdusergeneratordir = /PREFIX/lib/systemd/user-generators
systemdsystemgeneratordir = /ROOTPREFIX/lib/systemd/system-generators
systemduserunitpath = /SYSCONF/systemd/user:/etc/systemd/user:/run/systemd/user:/usr/local/lib/systemd/user:/usr/local/share/systemd/user:/PREFIX/lib/systemd/user:/usr/lib/systemd/user:/usr/share/systemd/user
systemdsystemunitpath = /SYSCONF/systemd/system:/etc/systemd/system:/run/systemd/system:/usr/local/lib/systemd/system:/ROOTPREFIX/lib/systemd/system:/usr/lib/systemd/system:/lib/systemd/system
systemduserconfdir = /SYSCONF/systemd/user
systemdsystemconfdir = /SYSCONF/systemd/system
systemduserpresetdir = /PREFIX/lib/systemd/user-preset
systemduserunitdir = /PREFIX/lib/systemd/user
systemdsystempresetdir = /ROOTPREFIX/lib/systemd/system-preset
systemdsystemunitdir = /ROOTPREFIX/lib/systemd/system
systemdutildir = /usr/lib/systemd
sysconfdir = /SYSCONF
rootprefix = /ROOTPREFIX
prefix = /PREFIX
pcfiledir = /usr/share/pkgconfig

(pkgconf doesn't provide a way to print all variables together with their definitions,
according to the man page. Disappointing.)