bug#15094: Fix for posix_memalign on Cygwin

[Ken Brown <kbrown@cornell.edu>]

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Cygwin allows applications to supply their own malloc but not, until 
today, their own posix_memalign.  This has caused problems with the GTK 
build of Emacs on Cygwin.  The problem became worse with the latest Glib 
update.  Anyone who wants the gory details can find them starting here:

   http://cygwin.com/ml/cygwin-xfree/2013-08/msg00018.html

Cygwin was just patched today to allow applications to supply their own 
posix_memalign (but not their own memalign), which solves the Glib 
problem.  But posix_memalign in gmalloc.c calls memalign, so I need to 
make sure that it calls Emacs's memalign.  The attached patch does this. 
  Is this a reasonable way to handle the problem?  (I plan to also add 
an explanatory comment.)

Thanks.

Ken

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=== modified file 'src/gmalloc.c'
--- src/gmalloc.c	2013-01-02 16:13:04 +0000
+++ src/gmalloc.c	2013-08-14 16:01:02 +0000
@@ -1558,8 +1558,14 @@
 
 void *(*__memalign_hook) (size_t size, size_t alignment);
 
+#ifdef CYGWIN
+#define memalign memalign1
+void *
+memalign1 (size_t alignment, size_t size)
+#else
 void *
 memalign (size_t alignment, size_t size)
+#endif
 {
   void *result;
   size_t adj, lastadj;