meson: do not use source_root() when possible

source_root() function is deprecated in Meson version 0.56.0 because
it returns the source root of the parent project if called from a
subproject.

Why would anyone need Mesa as a meson subproject?
It would be used as subproject in a project generated by command buffer
"decompiler" for Freedreno.

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Reviewed-by: Eric Engestrom <eric@igalia.com>
Reviewed-by: David Heidelberg <david.heidelberg@collabora.com>
Acked-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19901>

bin/meson_get_version.py

@@ -20,11 +20,16 @@
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
+import argparse
 import os
 
 
 def main():
-    filename = os.path.join(os.environ['MESON_SOURCE_ROOT'], 'VERSION')
+    parser = argparse.ArgumentParser()
+    parser.add_argument('version_dir', help="Directory with VERSION file")
+    args = parser.parse_args()
+
+    filename = os.path.join(args.version_dir, 'VERSION')
     with open(filename) as f:
         version = f.read().strip()
     print(version, end='')