configure: --with-pythondir should also override pyexecdir

Fixes #512.

* configure.ac: Here.
* NEWS: Mention the bug.

NEWS

@@ -15,6 +15,10 @@ New in spot 2.11.2.dev  (not yet released)
     during automata-based implication checks) would leak memory on
     abort.
 
+  - configure --with-pythondir should also redefine pyexecdir,
+    otherwise, libraries get installed in the wrong place on Debian.
+    (Issue #512.)
+
 New in spot 2.11.2  (2022-10-26)
 
   Command-line tools:

configure.ac

@@ -192,7 +192,7 @@ if test "x${enable_python:-yes}" = xyes; then
 
   AC_ARG_WITH([pythondir],
   [AS_HELP_STRING([--with-pythondir], [override the computed pythondir])],
-  [pythondir=$withval], [])
+  [pythondir=$withval pyexecdir=$withval], [])
 fi