CRM-20169 Added alterReportVar hook support to activity report

----------------------------------------
* CRM-20169: Add support for alterReportVar hook in Activity Report
  https://issues.civicrm.org/jira/browse/CRM-20169

CRM/Report/Form/Activity.php

@@ -859,10 +859,9 @@ public function postProcess() {
       }
     }
     $this->limit();
-    $groupByFromSelect = CRM_Contact_BAO_Query::getGroupByFromSelectColumns($this->_selectClauses, 'civicrm_activity_id');
-    $sql = "{$this->_select}
-FROM civireport_activity_temp_target tar
-{$groupByFromSelect} {$this->_having} {$this->_orderBy} {$this->_limit}";
+
+    $this->getSQLQuery($sql);
+
     $this->buildRows($sql, $rows);
 
     // format result set.
@@ -1107,4 +1106,19 @@ public function sectionTotals() {
     }
   }
 
+  /**
+   * Function to add alterReportVar hook support.
+   *
+   * @param string $sql
+   *  The final SQL required to build the report.
+   **/
+  public function getSQLQuery(&$sql) {
+    $this->_from = " FROM civireport_activity_temp_target tar ";
+    $this->_groupBy = CRM_Contact_BAO_Query::getGroupByFromSelectColumns($this->_selectClauses, 'civicrm_activity_id');
+
+    CRM_Utils_Hook::alterReportVar('sql', $this, $this);
+
+    $sql = "{$this->_select} {$this->_from} {$this->_where} {$this->_groupBy} {$this->_having} {$this->_orderBy} {$this->_limit}";
+  }
+
 }