configure.ac: disable sphinx_basic_ng with --disable-doc

The sphinx_basic_ng package is solely a dependency of furo, which is
already disabled with --disable-doc. We should therefore disable
sphinx_basic_ng with --disable-doc as well.

Author: orlitzky

configure.ac

@@ -524,7 +524,7 @@ AC_ARG_ENABLE([doc],
   AS_HELP_STRING([--disable-doc],
                  [disable build of the Sage documentation and packages depending on it]), [
     dnl Disable packages needed for docbuilding
-    for pkg in sage_docbuild alabaster babel snowballstemmer imagesize sphinx sphinxcontrib_devhelp sphinxcontrib_jsmath sphinxcontrib_serializinghtml sphinxcontrib_applehelp sphinxcontrib_htmlhelp sphinxcontrib_qthelp sphinxcontrib_websupport jupyter_sphinx furo sphinx_copybutton mathjax; do
+    for pkg in sage_docbuild alabaster babel snowballstemmer imagesize sphinx sphinxcontrib_devhelp sphinxcontrib_jsmath sphinxcontrib_serializinghtml sphinxcontrib_applehelp sphinxcontrib_htmlhelp sphinxcontrib_qthelp sphinxcontrib_websupport jupyter_sphinx furo sphinx_copybutton mathjax sphinx_basic_ng; do
       AS_VAR_SET([SAGE_ENABLE_$pkg], [$enableval])
     done
     AS_VAR_IF([enableval], [no], [dnl Disable the docbuild by disabling the install tree for documentation