build: meson: sync locations with autotools

Autotools, probably due to historic reasons, creates the following files:
/usr/lib/polkit-1/polkitd
/usr/lib/polkit-1/polkit-agent-helper-1
($prefix = /usr)

Before this patch meson used not /usr/lib/ but $libdir,
and $libdir is /usr/lib on a few Linux distros, e.g. Arch Linux,
but it is /usr/lib64 on many 64-bit Linux distros using biarch, including
ROSA, ALT, Fedora, SUSE etc., and is /usr/lib/$arch on multiarch systems (e.g. Debian).

So, switching from autotools to meson relocated /usr/lib/polkit-1/* to /usr/lib64/polkit-1/ on ROSA.
This relocation does not make sense to my mind, $libexecdir, which is /usr/libexec on ROSA and Fedora
and sometimes is /usr/lib (e.g. in ALT Linux), would be a better place for those executable binaries
(and also relocation may require adjusting SELinux policy).

Also, that probably breaks polkit because /usr/lib/polkit-1/ is hardcoded:
src/polkitagent/polkitagentsession.c:599:  helper_argv[0] = PACKAGE_PREFIX "/lib/polkit-1/polkit-agent-helper-1";

I suspected that some external programs may rely on /usr/lib/polkit-1/, but searching for "lib/polkit-1"
on http://codesearch.debian.net did not find such programs.

For now let's just use the old location in meson.
Later a switch to $libexecdir can be done.

Signed-off-by: Mikhail Novosyolov <m.novosyolov@rosalinux.ru>
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