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 36 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #6810862
meson android arm64 fedora

00:01:36

passed #6810860
gst-macos-10.15
meson macos

00:03:01

passed #6810857
meson-mingw-w64-build

00:04:26

manual #6810861
docker windows 1809 allowed to fail manual
meson msys2
passed #6810855
meson-shared-build

00:05:06

passed #6810856
meson-static-build

00:05:29

passed #6810858
docker windows 1809
meson vs2017 amd64

00:01:21

passed #6810859
docker windows 1809
meson vs2017 x86

00:01:19

passed #6810854
mingw-build

00:06:16

passed #6810852
shared-build

00:07:36

passed #6810853
static-build

00:04:38