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
1 file changed