configure: Add -Wmissing-declarations to maximum compiler warnings set

This helps to catch inconsistent declarations in headers that weren’t
included, and local functions that are not marked static.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

configure.ac

@@ -33,11 +33,11 @@ AC_ARG_ENABLE([compile-warnings],
        ;;
      maximum)
        WARNING_CXXFLAGS="-Wall"
-       PICKY_CXXFLAGS="-Wextra -pedantic -Wno-long-long -Weffc++"
+       PICKY_CXXFLAGS="-Wextra -pedantic -Wno-long-long -Weffc++ -Wmissing-declarations"
        ;;
      error)
        WARNING_CXXFLAGS="-Wall -Werror"
-       PICKY_CXXFLAGS="-Wextra -pedantic -Wno-long-long -Weffc++"
+       PICKY_CXXFLAGS="-Wextra -pedantic -Wno-long-long -Weffc++ -Wmissing-declarations"
        ;;
      *)
        AC_MSG_ERROR([Unknown argument '$enableval' to --enable-compile-warnings])