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/ltlast/Makefile.am

@@ -33,7 +33,6 @@ ltlast_HEADERS = \
   bunop.hh \
   constant.hh \
   formula.hh \
-  formula_tree.hh \
   multop.hh \
   nfa.hh \
   predecl.hh \
@@ -50,6 +49,7 @@ libltlast_la_SOURCES = \
   constant.cc \
   formula.cc \
   formula_tree.cc \
+  formula_tree.hh \
   multop.cc \
   nfa.cc \
   refformula.cc \