rename src/ as spot/ and use include <spot/...>

* NEWS: Mention the change.
* src/: Rename as ...
* spot/: ... this, adjust all headers to include <spot/...> instead of
"...", and adjust all Makefile.am to search headers from the top-level
directory.
* HACKING: Add conventions about #include.
* spot/sanity/style.test: Add a few more grep to catch cases
that do not follow these conventions.
* .gitignore, Makefile.am, README, bench/stutter/Makefile.am,
bench/stutter/stutter_invariance_formulas.cc,
bench/stutter/stutter_invariance_randomgraph.cc, configure.ac,
debian/rules, doc/Doxyfile.in, doc/Makefile.am,
doc/org/.dir-locals.el.in, doc/org/g++wrap.in, doc/org/init.el.in,
doc/org/tut01.org, doc/org/tut02.org, doc/org/tut03.org,
doc/org/tut10.org, doc/org/tut20.org, doc/org/tut21.org,
doc/org/tut22.org, doc/org/tut30.org, iface/ltsmin/Makefile.am,
iface/ltsmin/kripke.test, iface/ltsmin/ltsmin.cc,
iface/ltsmin/ltsmin.hh, iface/ltsmin/modelcheck.cc,
wrap/python/Makefile.am, wrap/python/ajax/spotcgi.in,
wrap/python/spot_impl.i, wrap/python/tests/ltl2tgba.py,
wrap/python/tests/randgen.py, wrap/python/tests/run.in: Adjust.

NEWS

@@ -4,6 +4,22 @@ New in spot 1.99.6a (not yet released)
 
   Library:
 
+  * Installed headers now assume that they will be included as
+    #include <spot/subdir/header.hh>
+    instead of
+    #include <subdir/header.hh>
+
+    This implies that when Spot headers are installed in
+    /usr/include/spot/... (the default when using the Debian packages)
+    or /usr/local/include/spot/... (the default when compiling from
+    source), then it is no longuer necessary to add
+    -I/usr/include/spot or -I/usr/local/include/spot when compiling.
+
+    Inside the source distribution, the subdirectory src/ has been
+    renamed to spot/, so that the root of the source tree can also be
+    put on the preprocessor's search path to compile against a
+    non-installed version of Spot.
+
   Python:
 
   Bug fixes: