Configuration files such as curl_config.h and all config-*.h no longer exist
nor are generated/copied into 'src' directory, now these only exist in 'lib'
directory from where curl tool sources uses them.
Additionally old src/setup.h has been refactored into src/tool_setup.h which
now pulls lib/setup.h
The possibility of a makefile needing an include path adjustment exists.
Fix the str2num() function to not check if the input string starts with a
digit, since strtol() supports numbers prepended with '-' (and '+') too.
This makes the --max-redirs option work as documented.