Change behavior of calls for fork() on non-fork() platforms

Change behavior of calls for fork() on non-fork() platforms

It was requested by users of Windows (e.g. non-fork() supporting
platforms) that calls to set CK_FORK mode be ignored if fork()
is not supported. This is to avoid the need to detect the
current platform and conditionally compile unit test code.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@1202 64e312b2-a51f-0410-8e61-82d0ca0eb02a