====================
Empty if [condition]
====================
if ( cond )
endif()

---

(source_file
  (if_condition
    (if_command
      (if)
      (argument (unquoted_argument))
    )
    (endif_command (endif))
  )
)

===========================
Empty if elseif [condition]
===========================
if(cond)
elseif(cond)
endif()

---

(source_file
  (if_condition
    (if_command
      (if)
      (argument (unquoted_argument))
    )
    (elseif_command
      (elseif)
      (argument (unquoted_argument))
    )
    (endif_command (endif))
  )
)

================================
Empty if elseif else [condition]
================================
if(cond)
elseif(cond)
else()
endif()

---

(source_file
  (if_condition
    (if_command
      (if)
      (argument (unquoted_argument))
    )
    (elseif_command
      (elseif)
      (argument (unquoted_argument))
    )
    (else_command (else))
    (endif_command (endif))
  )
)

==========================================
If with one command invocation [condition]
==========================================
if(cond)
  message(STATUS)
endif()

---

(source_file
 (if_condition
  (if_command
   (if)
   (argument (unquoted_argument))
  )
  (normal_command
   (identifier)
   (argument (unquoted_argument))
  )
  (endif_command (endif))
 )
)