$(top_builddir)/lib should also be in the include path

[Ben Pfaff]

The gnulib manual notes that $(top_srcdir)/lib should be in the
include path:

    Finally, you have add C flags and LD flags, so that you can make
    use of the gnulib library. For example:

         ...
         AM_CPPFLAGS = -I$(top_srcdir)/lib
         ...

However, it neglects to add that, because modules such as getopt
may copy files into the build directory, $(top_builddir)/lib
should also be added to the include path.
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 rien ne me parut plus important qu'un livre.
 La biblioth`eque, j'y voyais un temple."
--Jean-Paul Sartre