rrdtool: bump version to 1.6.0

rrdtool: bump version to 1.6.0

 - Remove patch
   0001-Add-configure-option-to-disable-documentation.patch as this is
   now include in the new version.

 - Add dependency on gettext when needed, since it uses libintl if
   gettext functionality is not provided by the C library.

 - Keep autoreconf/gettextize, which are needed for the link against
   libintl to work properly.

Signed-off-by: Roland Franke <fli4l@franke-prem.de>
[Thomas:
 - add dependency on gettext when needed
 - add comment about why autoreconf/gettextize are kept
 - cleanup commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>