Add a sanity check for installed private headers.

If an installed header has an associated *.cc file (in the source
tree), but does not declare any SPOT_API symbol, something is fishy.
Either that header should not be installed, or it is missing the
SPOT_API markers.

* src/sanity/private.test: New test.
* src/sanity/Makefile.am: Call it.
* src/ltlast/Makefile.am: Do not install formula_tree.hh.
* src/ltlvisit/Makefile.am: Do not install mark.hh.
* src/tgbaalgos/Makefile.am: Do not intall weight.hh.

src/ltlvisit/Makefile.am

@@ -35,7 +35,6 @@ ltlvisit_HEADERS = \
   lbt.hh \
   length.hh \
   lunabbrev.hh \
-  mark.hh \
   nenoform.hh \
   postfix.hh \
   randomltl.hh \
@@ -61,6 +60,7 @@ libltlvisit_la_SOURCES = \
   length.cc \
   lunabbrev.cc \
   mark.cc \
+  mark.hh \
   nenoform.cc \
   postfix.cc \
   randomltl.cc \