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.