meson: Don't harcode path to site-packages

Ask sysconfig instead for the right path. This fixes the default
value under MSYS2.
31 jobs for !230 with meson-no-site-hardcode in 4 minutes and 18 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Build Docker
passed #7464997
alpine amd64 manifest builder docker

00:00:17

passed #7464996
android docker

00:02:52

passed #7464995
cerbero fedora amd64 docker

00:03:06

passed #7464994
fedora amd64 docker

00:04:07

passed #7464998
gst-indent amd64 docker

00:00:16

 
  Preparation
passed #7465001
check allow-edit

00:00:12

passed #7465000
gst indent

00:00:10

manual #7464999
manual
manifest
 
  Build
created #7465010
gstreamer
build cerbero cross-android universal
created #7465014
gst-ios-14.3
build cerbero cross-ios universal
created #7465011
gstreamer
build cerbero cross win32
created #7465012
gstreamer
build cerbero cross win64
created #7465009
gstreamer
build cerbero fedora x86_64
created #7465013
gst-macos-11.1
build cerbero macos x86_64
created #7465015
docker windows gstreamer-windows 1809
build cerbero msvc x86_64
created #7465005
build clang fedora x86_64
created #7465008
docker windows 1809 allowed to fail manual
build msys2
created #7465002
build nodebug fedora x86_64
created #7465003
build static fedora x86_64
created #7465004
build static nodebug fedora x86_64
created #7465006
docker windows 1809
build vs2017 amd64
created #7465007
docker windows 1809
build vs2017 x86
 
  Test
created #7465016
gstreamer
check fedora
created #7465017
gstreamer
integration testsuites fedora 1/4
created #7465018
gstreamer
integration testsuites fedora 2/4
created #7465019
gstreamer
integration testsuites fedora 3/4
created #7465020
gstreamer
integration testsuites fedora 4/4
created #7465021
gstreamer
valgrind ges
 
  Integrate
created #7465022
cross-android universal examples
created #7465024
gst-ios-14.3
cross-ios universal examples
created #7465023
documentation