Fix in-src-tree builds by making gdbserver/gnulib/ a separate library (a la libiberty, etc.), and adding ACX_CONFIGURE_DIR.

[Joel Brobecker]

> GNULIB_CFLAGS=-I$(srcdir)/gnulib/import -I$(GNULIB_BUILDDIR)/import
> 
> isn't that confusing.  $(GNULIB_BUILDDIR)/import is the build of
> the gnulib import.
> 
> Would that make everybody happy?

That would be a good naming choice, indeed.

-- 
Joel