Fix latex_path on cygwin:

Fix latex_path on cygwin:
        * src/support/os.h
        * src/support/os_unix.C
        * src/support/os_win32.C
        * src/support/os_cygwin.C
        (latex_path): new function, handle the OS-specific part of
        lyx::support::latex_path

        * src/support/filetools.C
        (latex_path): use lyx::os::support::latex_path (bug 2409)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13474 a592a061-630c-0410-9148-cb99ea01b6c8