Adding a configure.ac in lib/ and using AC_CONFIG_SUBDIR

[Simon Josefsson]

I've run into some problems with gnulib for mingw, and the gnulib
recommended approach is that you need one config.h per gnulib-directory.

Right now we have lgl/ for the core libgnutls, libextra/gl/ for
libgnutls-extra, and gl/ for tools and self-tests.  So what is needed is
to move the libgnutls-specific configure.in code into a
lib/configure.ac, and the libgnutls-extra specific configure.in code
into a libextra/configure.ac.  This will make 'autoreconf' and
'configure' take slightly longer, but it allows us to cleanly separate
the projects various tasks, and will make gnulib module work again.

I think it is good timing to make this change early in the current 2.7.x
branch.

Any thoughts or objections?

/Simon