security/sssd: Fix installation with automake 1.16.4

Patch the configure script so it evaluates a variable that uses
PYTHON_PREFIX (since automake 1.16.4) before PYTHON_PREFIX is unset.

PR: 258898
Approved by: maintainer timeout (2 weeks)

(cherry picked from commit b07a361a29711f26c51b24b433662baa37648194)