Properly support Apache 2.2 and 2.4

Can be done by using <IfModule> directive.
Tested on fc20, el7 and Wheezy.

refs #6771

etc/apache/icingaweb.conf.in

@@ -3,10 +3,19 @@ Alias @web_path@ "@prefix@/public"
 <Directory "@prefix@/public">
     Options SymLinksIfOwnerMatch
     AllowOverride None
+
+    <IfModule mod_authz_core.c>
+        # Apache 2.4
+        <RequireAll>
+            Require all granted
+        </RequireAll>
+    </IfModule>
+
+    <IfModule !mod_authz_core.c>
+        # Apache 2.2
         Order allow,deny
         Allow from all
-    # new directive needed in Apache 2.4.3+
-    #Require all granted
+    </IfModule>
 
     SetEnv ICINGAWEB_CONFIGDIR @icingaweb_config_path@