Compile 3rdparty/hunspell in stdlib-debug mode when needed

Compile 3rdparty/hunspell in stdlib-debug mode when needed

A function that returns a vector<string> will lead to a crash if the
main code and the library have not been compiled with the same
stdlib-debug state. See for example:
https://stackoverflow.com/questions/4764048/stl-and-release-debug-library-mess

This is fixed by introducing a new variable STDLIB_DEBUG that contains
the flags that trigger the debug mode (autoconf only for now).

This will allow to go forward with bug #10547.