Quote macro argument to AC_ARG_WITH()

On some autoconf versions (e.g. 2.71 on MacOS, installed with `brew 
install autoconf`), not quoting this argument results in a syntax error
in `configure`:

```
sh configure
# ...
checking for joinable pthread attribute... PTHREAD_CREATE_JOINABLE
checking whether more special flags are required for pthreads... no
checking for PTHREAD_PRIO_INHERIT... yes
configure: line 14927: syntax error near unexpected token `newline'
configure: line 14927: `    '''
```

Adding `[...]` quotes fixes this; see 
https://gerrit.openafs.org/#/c/9995/ for a similar problem in a
different OSS project.

configure.ac

@@ -32,7 +32,7 @@ AC_ARG_WITH([rst2man],
 		[--with-rst2man=PATH],
 		[Location of rst2man (auto)]),
 	[RST2MAN="$withval"],
-	AC_CHECK_PROGS(RST2MAN, [rst2man rst2man.py], []))
+	[AC_CHECK_PROGS(RST2MAN, [rst2man rst2man.py], [])])
 AM_CONDITIONAL(HAVE_RST2MAN, [test "x$RST2MAN" != "xno"])
 
 m4_ifndef([VARNISH_PREREQ], AC_MSG_ERROR([Need varnish.m4 -- see README.rst]))