Refactor of asksource/config process so that variable defs are written to .defs files

Server/src/make.defs is included in MAkefile; and similarly for do_compile.defs / Server/src/do_compile.sh.

This increases the reliability of the asksource.sh config process by not needing to do in-file pattern replacement with sed.

(This became a problem when using mysql_config to generate build parameters for MySQL, part of refactoring MySQL support to be more robust.)