fix cygwin port

Hi,

long time no see, and im back. :smile: 

after applied these commits, cygwin port will revive.
src/nvc.exe can be build now, but it has undefined reference errors on bootstrapping under lib/std/std/STD.TEXTIO.dll.

```
$ make
Making all in lxt
Making all in fst
Making all in src
Making all in rt
Making all in lib
Making all in std
  NVC      std/STD.STANDARD
  NVC      std/STD.TEXTIO
  NATIVE   std/STD.TEXTIO
/tmp/ccRNyzrn.o:fake:(.text+0x2b): undefined reference to `_file_open'
/tmp/ccRNyzrn.o:fake:(.text+0x57): undefined reference to `_file_open'
/usr/lib/gcc/i686-pc-cygwin/4.8.2/../../../../i686-pc-cygwin/bin/ld: /tmp/ccRNyzrn.o: bad reloc address 0x20 in section `.eh_frame'
collect2: error: ld returned 1 exit status
** Fatal: /usr/bin/gcc failed with status 1
Makefile:499: recipe for target 'std/STD.TEXTIO' failed
```

IMHO, to support native code generation on cygwin, Id have to do one of following ways:
* use cygwin dll black magic
http://stackoverflow.com/questions/9317822/undefined-reference-when-dynamically-linking-with-gcc-in-cygwin
* use libtool w/ -no-undefined
https://www.sourceware.org/autobook/autobook/autobook_88.html

any thoughts?

Peace,