This rewrites the keyword/argument parsing and handling in the
AddExternalProject module to use arguments more literally:
- The strict keyword-value pairing is gone in favor of keywords with
arbitrary non-keyword values. This avoids requiring users to escape
spaces and quotes in command lines.
- Customized step command lines are now specified with a single
keyword <step>_COMMAND instead of putting the arguments in a
separate entry (previously called <step>_ARGS).
- Build step custom commands now use VERBATIM mode so that arguments
are correctly escaped on the command line during builds.
6ae0ff626a