cmake: fix HAVE_GETHOSTNAME definition

Otherwise Curl_gethostname always fails. Windows has gethostname
since Vista according to
http://msdn.microsoft.com/en-us/library/ms738527%28VS.85%29.aspx, but
accordings to byte_bucket's VC 2005 documentation, it is available even
in Windows 95. (possibly after installing a Platform SDK, the
Windows Server 2003 SP1 Platform SDK should be sufficient).

Signed-off-by: Peter Wu <peter@lekensteyn.nl>