Add support for XDG_DATA_DIRS

Add dirs from XDG_DATA_DIRS when <dir prefix="xdg"> appears in fonts.conf

Fixes fontconfig/fontconfig#271
12 jobs for issue/271 in 12 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #8409471
meson android arm64 fedora

00:01:43

passed #8409469
gst-macos-10.15
meson macos

00:04:20

passed #8409477
meson-mingw-w64-build

00:04:34

manual #8409470
docker windows 1809 allowed to fail manual
meson msys2
passed #8409464
meson-shared-build

00:05:35

passed #8409465
meson-static-build

00:05:56

passed #8409467
docker windows 1809
meson vs2017 amd64

00:01:29

passed #8409468
docker windows 1809
meson vs2017 x86

00:01:31

passed #8409463
mingw-build

00:06:07

passed #8409461
shared-build

00:07:59

passed #8409462
static-build

00:05:46

failed #8409466
allowed to fail
meson-mingw-w64-build

00:05:35