Echo stderr from captured command.

This helps with debugging if the command fails.

mesonbuild/scripts/meson_exe.py

@@ -63,6 +63,8 @@ def run_exe(exe):
     if exe.capture and p.returncode == 0:
         with open(exe.capture, 'wb') as output:
             output.write(stdout)
+    if stderr:
+        sys.stderr.buffer.write(stderr)
     return p.returncode
 
 def run(args):