[patch] to build docs on Mac OS X El Capitan with MacPorts

The attached patch was required to get the docs to build on Mac OS X 10.11.1 (15B42) with MacPorts 2.3.4.  After
changingdocbook.m4 ‘autoreconf’ has to be run.  This patch does not include the new version of ‘configure' 

As explained in the devel documentation (J.2.6) the docs could be made to build by adding ‘export
DOCBOOKSTYLE=/opt/local/share/dsssl/docbook-dsssl'

However, in J.2.4 it implies that just using MacPorts will work without additional setup.  This patch makes that
happen.

diff --git a/config/docbook.m4 b/config/docbook.m4
index 4304fa7..f88889d 100644
--- a/config/docbook.m4
+++ b/config/docbook.m4
@@ -55,7 +55,7 @@ AC_CACHE_VAL([pgac_cv_path_stylesheets],[if test -n "$DOCBOOKSTYLE"; then
pgac_cv_path_stylesheets=$DOCBOOKSTYLEelse
-  for pgac_prefix in /usr /usr/local /opt /sw; do
+  for pgac_prefix in /usr /usr/local /opt /opt/local /sw; do    for pgac_infix in share lib; do      for pgac_postfix
in\        sgml/stylesheets/nwalsh-modular \ 
@@ -65,7 +65,8 @@ else        sgml/docbook/dsssl/modular \        sgml/docbook/stylesheet/dsssl/modular \
sgml/docbook/dsssl-stylesheets\ 
-        sgml/dsssl/docbook-dsssl-nwalsh
+        sgml/dsssl/docbook-dsssl-nwalsh \
+        dsssl/docbook-dsssl      do        pgac_candidate=$pgac_prefix/$pgac_infix/$pgac_postfix        if test -r
"$pgac_candidate/html/docbook.dsl"\