Fix following error on running configure.

configure: WARNING: `missing' script is too old or missing

http://lists.gnu.org/archive/html/automake/2010-08/msg00114.html
Fix following error on cygwin.

In file included from Header.h:13:0,
                 from Settings.h:15,
                 from AvailableMetersPanel.h:12,
                 from AvailableMetersPanel.c:8:
Meter.h:88:19: error: field $B!F(Btime$B!G(B has incomplete type
    struct timeval time;
                   ^
Emulate reasonable ioprio_{get,set} failure if sys/syscall.h is missing.
Use sigaction to be portable.
Fix for array subscript has type `char' on cygwin.

Ideally, C99 character handling ctype.h should have is*() and to*() as
function, not macro. But newlib doesn't.

When using ctype.h, calling with int argument made sources portable in
practice.

Or, newlib should be fixed ASAP.
Remove unneeded return value stub for ioprio_set if sys/syscall.h is missing.
Regen.
Update htop_CFLAGS for warning free.
Merge pull request #1 from hiyuh/cygwin

Cygwin