fc-scan: add --sysroot option

and fix scanning to get things work with sysroot

Fixes fontconfig/fontconfig#225
10 jobs for issue/225 in 6 minutes and 36 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #4538588
gst-macos-10.15
meson macos

00:00:49

passed #4538585
meson-mingw-w64-build

00:05:57

manual #4538589
docker windows 1809 allowed to fail manual
meson msys2
passed #4538583
meson-shared-build

00:05:28

passed #4538584
meson-static-build

00:05:03

passed #4538586
docker windows 1809
meson vs2017 amd64

00:01:05

passed #4538587
docker windows 1809
meson vs2017 x86

00:01:05

passed #4538582
mingw-build

00:06:32

passed #4538580
shared-build

00:06:35

passed #4538581
static-build

00:05:50