Use -fvisibility=hidden globally.

* configure.ac: Check for flags and fill CXXFLAGS and CFLAGS.
* iface/dve2/dve2.hh: Mark load_dve2 for export.
* src/eltlparse/Makefile.am, src/kripke/Makefile.am,
src/kripkeparse/Makefile.am, src/ltlast/Makefile.am,
src/ltlenv/Makefile.am, src/ltlparse/Makefile.am,
src/ltlvisit/Makefile.am, src/misc/Makefile.am,
src/neverparse/Makefile.am, src/priv/Makefile.am, src/saba/Makefile.am,
src/sabaalgos/Makefile.am, src/ta/Makefile.am, src/taalgos/Makefile.am,
src/tgba/Makefile.am, src/tgbaalgos/Makefile.am,
src/tgbaalgos/gtec/Makefile.am, src/tgbaparse/Makefile.am:
Remove $(VISIBILITY_CXXFLAGS) now that it is set globally.

src/ltlast/Makefile.am

@@ -21,7 +21,7 @@
 ## along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 AM_CPPFLAGS = -I$(srcdir)/.. -I..
-AM_CXXFLAGS = $(WARNING_CXXFLAGS) $(VISIBILITY_CXXFLAGS)
+AM_CXXFLAGS = $(WARNING_CXXFLAGS)
 
 ltlastdir = $(pkgincludedir)/ltlast