build: Handle 'environ' global variable differently on cygwin x86_64

Amendment to commit c97dafde.
* tests/test-environ.c.diff: New file.
* Makefile.am (EXTRA_DIST): Add new patch.

diff --git a/gnulib-local/tests/test-environ.c.diff b/gnulib-local/tests/test-environ.c.diff
new file mode 100644 (file)
index 0000000..6cc8f68
--- /dev/null
+++ b/gnulib-local/tests/test-environ.c.diff
@@ -0,0 +1,18 @@
+diff --git a/test-environ.c b/test-environ.c
+index 46a2eab..8afecb7 100644
+--- a/test-environ.c
++++ b/test-environ.c
+@@ -22,6 +22,13 @@
+ 
+ #include <string.h>
+ 
++/* environ is the exported symbol referencing the internal
++   __cygwin_environ variable on cygwin64:
++   <https://cygwin.com/ml/cygwin/2013-06/msg00228.html>.  */
++#if defined __CYGWIN__ && defined __x86_64__
++extern DLL_VARIABLE char **environ;
++#endif
++
+ int
+ main ()
+ {