ENH: provide wcleanBuild and improve wcleanPlatform

- new behaviour is to do nothing if no platform was specified.
  This helps avoid inadvertently removing files.

- support special platforms for compatibility with wmake/wclean targets.
  Eg, "wcleanPlatform all"

- allow use from ThirdParty top-level as well, since the directory
  structure is similar.

BUG: fix regression in wcleanLnIncludeAll introduced by 63bb0f78