Rework Open Watcom make files to use standard Wmake features

Remove slash/backslash problem, now only slashes are used,
Wmake automaticaly translate slash/backslash to proper version or tools are not sensitive for it.
Enable spaces in path.
Use internal rm command for all host platforms
Add error message if old Open Watcom version is used. Some old versions exhibit build problems for Curl latest version. Now only versions 1.8, 1.9 and 2.O beta are supported