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
NSS: avoid compiler warnings when built without http2 support
x509asn: moved out Curl_verifyhost from NSS builds

... as it isn't used then!
configure: call it GSS-API

... since that’s how the RFC calls it.
hostcheck: Curl_cert_hostcheck is not used by NSS builds
hostcheck: update comment after previous change
multi_runsingle: move timestamp into INIT

Setting the TIMER_STARTSINGLE timestamp first in CONNECT has the
drawback that for actions that go back to the CONNECT state, the time
stamp is reset and for the multi_socket API there's no corresponding
Curl_expire() then so the timeout logic gets wrong!

Reported-by: Brad Spencer
Bug: http://curl.haxx.se/mail/lib-2014-02/0036.html