Build docs automatically if xsltproc is working

m4/moo-docs.m4

@@ -1,9 +1,58 @@
+AC_DEFUN([_MOO_AC_CHECK_XSLT_DOCBOOK],[
+  AC_ARG_VAR([XSLTPROC],[path to xsltproc utility])
+  AC_CHECK_PROG([XSLTPROC],[xsltproc],[xsltproc])
+
+  if test -z "$XSLTPROC"; then
+    $2
+  fi
+
+  if test -n "$XSLTPROC"; then
+    AC_MSG_CHECKING([whether xsltproc works])
+    cat > conftest.docbook << EOFEOF
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
+                    "http://www.oasis-open.org/docbook/xml/4.4/docbookx.dtd"
+>
+<article>
+<articleinfo>
+<title>Title</title>
+</articleinfo>
+<sect1>
+<title>A section</title>
+<para>Blah blah blah.</para>
+</sect1>
+</article>
+EOFEOF
+    if ($XSLTPROC http://docbook.sourceforge.net/release/xsl/current/html/docbook.xsl conftest.docbook 2>/dev/null >/dev/null); then
+      AC_MSG_RESULT([yes])
+      rm -f conftest.docbook
+      $1
+    else
+      AC_MSG_RESULT([no])
+      rm -f conftest.docbook
+      $2
+    fi
+  fi
+])
+
 AC_DEFUN([MOO_DOCS],[
   AC_REQUIRE([MOO_AC_SET_DIRS])
 
   AC_ARG_ENABLE([help],
-    AC_HELP_STRING(--enable-help, [build html help files (default = NO). Will not work on all systems]),
-    [:],[enable_help=no])
+    AC_HELP_STRING(--disable-help, [Disable building html help files (default = auto).]),
+    [:],[enable_help=auto])
+
+  if test "x$enable_help" = xauto -o "x$enable_help" = xyes; then
+    _MOO_AC_CHECK_XSLT_DOCBOOK([_moo_xslt_working=yes],[_moo_xslt_working=no])
+    if test $_moo_xslt_working = yes; then
+      enable_help=yes
+    elif test "x$enable_help" = xauto; then
+      enable_help=no
+    else
+      AC_MSG_ERROR([xsltproc will not work])
+    fi
+  fi
+
   AM_CONDITIONAL(MOO_ENABLE_HELP, test "x$enable_help" = xyes)
   if test "x$enable_help" = xyes; then
     AC_DEFINE(MOO_ENABLE_HELP, [1], [enable help functionality])