- 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 9e2e111518
