Remove ancient W/A for cygwin.

This W/A was for issue 5.
https://github.com/nickg/nvc/issues/5

But src/rt/signal.h was gone.
https://github.com/nickg/nvc/commit/b99b6b1430ce471044c8c123331605161690a1d5
Remove largest suffix from CC for SYSTEM_CC.

CC may contain -std=... option.
This shell trick is dash-compatible.
Update m4.

Copy from GNU Autoconf Archive git master.
FYI, ax_check_gnu_make.m4 is missing on cygwin.
Fix following error on cygwin.

util.c:546:13: error: `is_debugger_running' defined but not used [-Werror=unused-function]
Fix following errors on cygwin.

elab.c:832:10: error: array subscript has type `char' [-Werror=char-subscripts]

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

When using ctype.h,

 * casting to int
 * do not use unary ops

made sources portable in practice.

Or, newlib should be fixed ASAP.
Define SO_EXT on configure.

This is first step to support native code generation on cygwin.
Update .gitignore.
Merge pull request #20 from hiyuh/cygwin

fix cygwin port