No `libreadwrite_twoints.so` generated by cmake

[QZ4869]

I make the with_cmake example(the detail I described in this issue).
It generated a file libreadwrite_twoints.rootmap, the content of it is

[ libreadwrite_twoints.so ]
# List of selected classes
class TwoInts
header twoints.hpp

but the libreadwrite_twoints.so is not in these directory.

we can use ROOT prompt to test this rootmap and dictionary, and get this error:

root -l
root [0] TwoInts a
Error in <TInterpreter::TCling::AutoLoad>: failure loading library libreadwrite_twoints.so for TwoInts
Error in <TInterpreter::TCling::AutoLoad>: failure loading library libreadwrite_twoints.so for TwoInts
Error in <TInterpreter::TCling::AutoLoad>: failure loading library libreadwrite_twoints.so for TwoInts
Error in <TInterpreter::TCling::AutoLoad>: failure loading library libreadwrite_twoints.so for TwoInts
ROOT_prompt_0:1:1: error: unknown type name 'TwoInts'
TwoInts a
^
root [1]