When a button is created, the class ui-corner-all is always applied. I tried the following: 
<p:commandButton id="like" styleClass="ui-corner-right" />
but it didn't work. On Firebug, I saw both ui-corner-all and ui-corner-right:
<div id="form1:like" type="button" class="ui-button ui-widget ui-state-default ui-corner-all ui-button-text-icon-left ui-corner-right">
UPDATE 1:
Following the hint from JMelnik, I finally succeeded to change the style of ui-corner-all to ui-corner-right by adding the following script:
<style type="text/css">
    #myForm\:likeButton .ui-corner-all {
        border-radius: 6px 0px 0px 6px !important;
    }
</style>
and wrap the <p:commandButton> inside <h:panelGroup> as following:
<h:form id="myForm">
    <h:panelGroup id="likeButton">
        <p:commandButton />
    <h:panelGroup>
</h:form>
UPDATE 2:
Thanks to BalusC's suggestion, the following solution should be better:
<style type="text/css">
    .likeButton .ui-corner-all {
        border-radius: 6px 0px 0px 6px !important;
    }
</style>  
<h:panelGroup styleClass="likeButton">
    <p:commandButton />
<h:panelGroup>
Best regards,
 
    