build: Handle 'environ' global variable differently on cygwin x86_64
Problem reported by Vasyl Khalak in:
<https://cygwin.com/ml/cygwin/2013-06/msg00228.html>.
* lib/execute.c.diff: New file.
* lib/spawn-pipe.c.diff: Likewise.
* Makefile.am (EXTRA_DIST): Add new patches.
