Handle absolute sysconfdir when installing symlinks

sysconfdir defaults to /etc when the prefix is set to /usr. But joining
MESON_INSTALL_DESTDIR_PREFIX and sysconfdir when the latter is an
absoulte path, results in sysconfdir only. Which might lead to an error
during install because /etc/fonts/conf.d/ might already exist from an
pre-existing fontconfig installation.
11 jobs for absolute-sysconfdir-in-install-script in 7 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #7710937
meson android arm64 fedora

00:02:06

passed #7710935
gst-macos-10.15
meson macos

00:04:01

passed #7710932
meson-mingw-w64-build

00:04:56

manual #7710936
docker windows 1809 allowed to fail manual
meson msys2
passed #7710930
meson-shared-build

00:05:03

passed #7710931
meson-static-build

00:05:50

passed #7710933
docker windows 1809
meson vs2017 amd64

00:01:25

passed #7710934
docker windows 1809
meson vs2017 x86

00:01:24

passed #7710929
mingw-build

00:06:25

passed #7710927
shared-build

00:07:26

passed #7710928
static-build

00:06:23