vim: Add gnumake and ninja extensions for builds.

  Update tests to reflect changes to extensions.
  Update config, extension logic.
  Refactor method to get makeprg program.
  Caught bug that caused bailing when a malformed target existed.
  Update unit test for target file discovery.
  Change logic on how target build directories are found.
  Changed command for GNU make's build.
vim: Validate support for Ninja in tests.