libcurl.m4: mention argument is PREFIX

The macro provides a --with-libcurl option that expects a PREFIX to be
specified and not actually a "directory" in which libcurl will be found.
This now spells that out more clearly.

Reported by: Dan Locks
Bug: http://curl.haxx.se/bug/view.cgi?id=3079891

docs/libcurl/libcurl.m4

@@ -61,7 +61,7 @@ AC_DEFUN([LIBCURL_CHECK_CONFIG],
   AH_TEMPLATE([LIBCURL_PROTOCOL_SMTP],[Defined if libcurl supports SMTP])
 
   AC_ARG_WITH(libcurl,
-     AC_HELP_STRING([--with-libcurl=DIR],[look for the curl library in DIR]),
+     AC_HELP_STRING([--with-libcurl=PREFIX],[look for the curl library in PREFIX/lib and headers in PREFIX/include]),
      [_libcurl_with=$withval],[_libcurl_with=ifelse([$1],,[yes],[$1])])
 
   if test "$_libcurl_with" != "no" ; then