Avoid recursive python3_sitearch expansion even if undefined

Fixes an error during SRPM construction if not enough dependencies
are installed (e.g., during the first build stage in mock):

error: /builddir/build/SPECS/samba.spec: line 2478: Too many levels
  of recursion in macro expansion. It is likely caused by recursive
  macro declaration.

samba.spec

@@ -194,8 +194,8 @@
 %global _systemd_extra "Environment=KRB5CCNAME=FILE:/run/samba/krb5cc_samba"
 
 # Make a copy of this variable to prevent repeated evaluation of the
-# embedded shell command.
-%global python3_sitearch %{python3_sitearch}
+# embedded shell command.  Avoid recursive macro definition if undefined.
+%{?python3_sitearch: %global python3_sitearch %{python3_sitearch}}
 
 Name:           samba
 Version:        %{samba_version}