cmake_minimum_required(VERSION 2.6)

macro(print var)
   message(STATUS "${var} = \"${${var}}\"")
endmacro(print)

set(FOO foo)
print(FOO)              # gives "foo"

find_program(FOO ls)    # this line doesn't affect the result at all
print(FOO)

mark_as_advanced(FOO)   # without this line FOO is still "foo" after the next find_program()
                        # with this line it is empty

print(FOO)              # gives "foo"

find_program(FOO ls)    # does nothing, since FOO is already valid
print(FOO)              # gives ""