gh-41260: Fix --with-python option not being honored when cache exists
    
When running ``./configure with --with-python=pythonX.Y``, if a previous
configure run had cached a different Python path in
``ac_cv_path_PYTHON3``,
the cached value would be used instead of re-checking the specified
Python.

This fix clears the cache variable when ``--with-python`` is explicitly
specified, forcing configure to always check the specified Python.

Fixes #41258

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URL: #41260
Reported by: Chenxin Zhong
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/python3/spkg-configure.m4

@@ -15,6 +15,13 @@ SAGE_SPKG_CONFIGURE([python3], [
          [AC_MSG_ERROR([building Sage --without-python is not supported])])
    ac_path_PYTHON3="$with_python"
 
+   dnl If --with-python was explicitly specified and differs from the cached value,
+   dnl clear the cache to force re-checking.
+   dnl This fixes issue #41258 where changing --with-python would use stale cached values
+   AS_IF([test x"$ac_path_PYTHON3" != x && test x"$ac_path_PYTHON3" != x"$ac_cv_path_PYTHON3"], [
+       AS_UNSET([ac_cv_path_PYTHON3])
+   ])
+
    dnl Issue #30559:  Removed the DEPCHECK for sqlite.  We never use libsqlite3 from SPKG for anything
    dnl other than building the python3 SPKG; so our libsqlite3 cannot create shared library conflicts.
    dnl