configure: fix typos in adl_CHECK_PYTHON

Fixes #220.

* m4/pypath.m4: Here.
* NEWS: Mention the bug.

NEWS

@@ -30,6 +30,10 @@ New in spot 2.3.0.dev (not yet released)
   - alternation_removal() was not always reporting the unsupported
     non-weak automata.
 
+  - a long-standing typo in the configure code checking for Python
+    caused any user-defined CPPFLAGS to be ignored while building
+    Spot.
+
 New in spot 2.3 (2017-01-19)
 
   Build:

m4/pypath.m4

@@ -10,7 +10,7 @@ AC_DEFUN([adl_CHECK_PYTHON],
 [sys.stdout.write(sysconfig.get_python_inc())" 2>/dev/null`])
   AC_SUBST([PYTHONINC], [$adl_cv_python_inc])
   adl_save_CPPFLAGS="$CPPFLAGS"
-  CPPFLAGS="$ac_save_CPPFLAGS -I$PYTHONINC"
+  CPPFLAGS="$adl_save_CPPFLAGS -I$PYTHONINC"
   AC_CHECK_HEADERS([Python.h],,
                    [AC_MSG_ERROR([Python's development headers are not installed.
 
@@ -20,5 +20,5 @@ to use Spot's Python bindings, you may also disable their compilation
 by running
   ./configure --disable-python
 and in this case you do not need python3-devel installed.])])
-  CPPFLAGS=$ac_save_CPPFLAGS
+  CPPFLAGS=$adl_save_CPPFLAGS
 ])