Update README for intltool to gettext migration (!107)

Remove mention of intltool as it's now unused.

Add polkit to the list of dependencies to build GParted from source as
gettext always explicitly translates the org.gnome.gparted.policy file.

Add polkit-devel and gettext-devel packages to the packages needing
installing on various distributions to get the gettext translation rules
for .policy files and the autopoint build tool installed.
(Distributions such as Debian and Ubuntu split the packages differently.
Gettext translation rules for .policy files are in the base policykit-1
package and the autopoint tool is in the autopoint package which I
assume is always installed as part of the development tool set.  Hence
no change to the command to install dependent packages on these
distributions.  See the earlier commit messages for more details).

Closes !107 - Migrate from intltool to gettext translation

README

@@ -93,13 +93,14 @@ b. Building from Source
       gnome-common
       gtkmm3
       gettext
-      intltool
+      polkit
       yelp-tools          - required if help documentation is to be built
 
    On CentOS/RHEL, these dependencies may be obtained by running the
    following command as root:
-      yum install gnome-common yelp-tools glib2-devel intltool gcc-c++ \
-                  libuuid-devel parted-devel gtkmm30-devel make
+      yum install gnome-common yelp-tools glib2-devel gcc-c++ \
+                  libuuid-devel parted-devel gtkmm30-devel make \
+                  polkit-devel gettext-devel
 
    On Debian or Ubuntu, these dependencies may be obtained by running
    one of the following commands:
@@ -112,13 +113,14 @@ b. Building from Source
 
    On Fedora, these dependencies may be obtained by running the
    following command as root:
-      dnf install gnome-common yelp-tools glib2-devel intltool gcc-c++ \
-                  libuuid-devel parted-devel gtkmm30-devel make
+      dnf install gnome-common yelp-tools glib2-devel gcc-c++ \
+                  libuuid-devel parted-devel gtkmm30-devel make \
+                  polkit-devel gettext-devel
 
    On openSUSE, these dependencies may be obtained by running the
    following commands:
       sudo zypper install gnome-common gcc-c++ libuuid-devel \
-                          parted-devel gtkmm3-devel make
+                          parted-devel gtkmm3-devel make polkit-devel
 
    Again, build GParted with the default configuration and install into
    the default location of /usr/local using: