use wxPOSIX_IDENT() for consistent handling of underscores in POSIX functions names...

use wxPOSIX_IDENT() for consistent handling of underscores in POSIX functions names in Windows compilers standard headers; this should hopefully fix the compilation for older Borland versions without breaking it for anything else; also define wxFsync() for Borland and DMC as they seem to have _commit() too (subject to version checks...), only Watcom seems to lack it

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@42615 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775