meson: fix subproject build regression

commit ae9ac2a1 from !165 introduced a regression when
fontconfig is built as a subproject. In that case we
would fail to correctly construct the path to the root
build dir where the meson-info subdirectory resides,
instead looking for it in the fontconfig subproject
subdir. This would result in
FAILED: subprojects/fontconfig/src/fcobjshash.gperf
errors in the cutout.py script.

Instead use the @BUILD_ROOT@ substitution to get
to the build root which will work correctly in
either scenario.
12 jobs for meson-fix-subproject-build in 13 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #7408975
meson android arm64 fedora

00:01:51

passed #7408973
gst-macos-10.15
meson macos

00:06:22

passed #7408970
meson-mingw-w64-build

00:07:09

manual #7408974
docker windows 1809 allowed to fail manual
meson msys2
passed #7408968
meson-shared-build

00:07:28

passed #7431197
meson-static-build

00:03:13

passed #7408971
docker windows 1809
meson vs2017 amd64

00:01:32

passed #7408972
docker windows 1809
meson vs2017 x86

00:01:31

passed #7408967
mingw-build

00:07:55

passed #7408965
shared-build

00:09:48

passed #7408966
static-build

00:07:48

failed #7408969
meson-static-build

00:06:55