Better way to set the reference documentation installation path

configure.ac

@@ -305,6 +305,8 @@ AC_SUBST([LIBCRYSTFEL_LIBS])
 GTK_DOC_CHECK([1.11],[--flavour no-tmpl])
 AC_CONFIG_MACRO_DIR(m4)
 
+HTML_DIR=${docdir}/reference
+
 
 AC_CONFIG_FILES(Makefile lib/Makefile doc/reference/Makefile libcrystfel/Makefile)
 AC_OUTPUT

m4/gtk-doc.m4

@@ -15,10 +15,9 @@ AC_DEFUN([GTK_DOC_CHECK],
   AC_PATH_PROG([GTKDOC_MKPDF],[gtkdoc-mkpdf])
 
   dnl for overriding the documentation installation directory
-  dnl Modified by TAW to customise default path
   AC_ARG_WITH([html-dir],
     AS_HELP_STRING([--with-html-dir=PATH], [path to installed docs]),,
-    [with_html_dir='${docdir}/reference'])
+    [with_html_dir='${datadir}/gtk-doc/html'])
   HTML_DIR="$with_html_dir"
   AC_SUBST([HTML_DIR])