configure: diagnose missing Python.h

Fixes #95, reported by Vitus Lam.

* m4/pypath.m4: Check for Python.h and print some advice if missing.
* NEWS: Mention this.
* THANKS: Add Vitus.

NEWS

 New in spot 1.99.1a (not yet released)
 
+  * A missing Python.h is now diagnosied at ./configure time,
+    with the suggestion to either install python3-devel, or
+    run ./configure --disable-python.
+
   * Bugs fixed:
     - p[+][:*2] was not detected as belonging to siPSL
     - scc_filter() would incorrectly remove Fin marks from

THANKS

@@ -24,4 +24,5 @@ Rüdiger Ehlers
 Silien Hong
 Sonali Dutta
 Tomáš Babiak
+Vitus Lam
 Yann Thierry-Mieg

m4/pypath.m4

@@ -8,4 +8,17 @@ AC_DEFUN([adl_CHECK_PYTHON],
     [adl_cv_python_inc],
     [adl_cv_python_inc=`$PYTHON -c "import sys; from distutils import sysconfig;]
 [sys.stdout.write(sysconfig.get_python_inc())" 2>/dev/null`])
-  AC_SUBST([PYTHONINC], [$adl_cv_python_inc])])
+  AC_SUBST([PYTHONINC], [$adl_cv_python_inc])
+  adl_save_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$ac_save_CPPFLAGS -I$PYTHONINC"
+  AC_CHECK_HEADERS([Python.h],,
+                   [AC_MSG_ERROR([Python's development headers are not installed.
+
+The package to install is often called python3-devel, but that name
+might be different in your distribution.  Note that if you do not plan
+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
+])