Fix m4 quoting on Sphinx check error message.

The error message quoting for the Sphinx check failure was removed in
c2d4999, but is actually needed.

If you omit the quoting, the error turns into this:

    checking for python3 script directory (pythondir)... ${PYTHON_PREFIX}/lib/python3.11/site-packages
    checking for python3 extension module directory (pyexecdir)... ${PYTHON_EXEC_PREFIX}/lib/python3.11/site-packages
    checking for ... /usr/local/bin/python3 -m sphinx
    checking for sphinx version >= 1.7.5... No module named 'sphinx'
    ./configure: line 441: test: check: integer expression expected
    configure: error: Sphinx
    ./configure: line 312: return: check: numeric argument required
    ./configure: line 322: exit: check: numeric argument required

Signed-off-by: James Peach <jpeach@apache.org>

Author: jpeach

configure.ac

@@ -362,7 +362,7 @@ AC_ARG_ENABLE([docs],
       ],[
         sphinx_version_check=no
         enable_doc_build=no
-        AC_MSG_ERROR(Doc building disabled, check Sphinx installation)
+        AC_MSG_ERROR([Doc building disabled, check Sphinx installation])
       ])