Disabling usage of readline header when only C++ API to avoid depending on libreadline-dev package

[titan73]

CMakeLists.txt reads READLINE_FOUND and sets ABC_USE_xxxx variables depending on TRUE & FALSE values.
I don't know how READLINE_FOUND is supposed to be set but if it is not, we are in a incoherent situation where the header is used but the librarie is not set.
In the case of OpenROAD where only the C++ API and readline is not used, the only way to prevent including the header is to set READLINE_FOUND to FALSE. There is maybe a better way to handle that.

[sarnold]

The workflows PR has updated cmake build bits and should set -DABC_USE_NO_READLINE appropriately. You can give that a try and see if it resolves your issue.