doc: give an example of using the macro for pkg-config

The autoconf manual doesn't mention any examples of actually
using m4_pattern_forbid.  Yet this is the perfect macro for
avoiding the all-too-common failure mode of configure dying with:

      checking if libxml2 is present... ./configure: line 11586: syntax
 error
 near unexpected token `LIBXML2,'
      ./configure: line 11586: `PKG_CHECK_MODULES(LIBXML2, libxml-2.0>=
 2.6.19,'

for developers that forgot to install pkg-config.  While we don't
necessarily advertise the use of PKG_CHECK_MODULES, it is a
common enough situation that the manual should make it easier to
help developers learn about missing third-party macros.

Based on a mailing list report by Daniel Pocock:
http://lists.gnu.org/archive/html/autoconf/2014-01/msg00030.html

* doc/autoconf.texi (Forbidden Patterns): Add examples.

Signed-off-by: Eric Blake <eblake@redhat.com>