Autogen: Restore AUTOGEN_PARALLEL support for numbers Refactoring in commit d867e05892 (Autogen: Use JSON instead of CMake script for info files, 2019-09-20, v3.16.0-rc1~57^2) broke support for numerical values of `AUTOGEN_PARALLEL`. Parse and verify the range. Warn if the value is invalid. Fixes: #20376