Enable -Wmissing-declarations in development mode.

* m4/gccwarn.m4: Add -Wmissing-declarations.
* iface/ltsmin/ltsmin.cc, iface/ltsmin/modelcheck.cc,
src/bin/common_trans.cc, src/bin/genltl.cc, src/bin/ltlgrind.cc,
src/tests/acc.cc, src/tests/bitvect.cc, src/tests/checkpsl.cc,
src/tests/checkta.cc, src/tests/complementation.cc,
src/tests/consterm.cc, src/tests/emptchk.cc, src/tests/equalsf.cc,
src/tests/graph.cc, src/tests/ikwiad.cc, src/tests/intvcmp2.cc,
src/tests/intvcomp.cc, src/tests/kind.cc, src/tests/length.cc,
src/tests/ltlprod.cc, src/tests/ltlrel.cc, src/tests/ngraph.cc,
src/tests/randtgba.cc, src/tests/readltl.cc, src/tests/reduc.cc,
src/tests/syntimpl.cc, src/tests/tostring.cc, src/tests/twagraph.cc,
src/tl/contain.cc, src/twaalgos/dtgbacomp.cc, src/twaalgos/minimize.cc:
Add "static" and move in anonymous namespace when appropriate.

m4/gccwarn.m4

@@ -32,7 +32,8 @@ EOF
    Wpointer-arith \
    Wwrite-strings \
    Wcast-qual \
-   Wdocumentation
+   Wdocumentation \
+   Wmissing-declarations
   do
     CXXFLAGS="$cf_save_CXXFLAGS $ac_cv_prog_gxx_warn_flags -$cf_opt"
     if AC_TRY_EVAL(ac_compile); then