Only define __getreent inline function when building newlib or Cygwin

Commit 6f3943b erroneously removed the `#ifdef _COMPILING_NEWLIB'
guarding the __getreent inline function.  This patch ignored the
fact that config.h is included when building applications, and the
code in question requires internal, auto-generated headers to be
available which are not exposed to user-space.

Reinstantiate defined(_COMPILING_NEWLIB) test and alternatively
check for defined (__INSIDE_CYGWIN__), otherwise we'd have to
reinstantiate the __getreent macro in cygtls.h which is really
confusing.

While testing it turned out that a low number of source codes inside
Cygwin won't see the inline __getreent due to a missing __INSIDE_CYGWIN__
definition.  For malloc.cc this was actually deliberate to get different
definitions from including cygmalloc.h.  Change this by defining
__INSIDE_CYGWIN__ in malloc.cc but changing the test in cygmalloc.h
to test for defined(DLMALLOC_VERSION).  This might need a change if we
ever get around to replace dlmalloc with a newer, more threading-aware
malloc implementation.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/cygmalloc.h

@@ -26,17 +26,22 @@ void dlmalloc_stats ();
 #define MALLOC_ALIGNMENT ((size_t)16U)
 #endif
 
-#ifndef __INSIDE_CYGWIN__
+#if defined (DLMALLOC_VERSION)	/* Building malloc.cc */
+
 extern "C" void __set_ENOMEM ();
 void *mmap64 (void *, size_t, int, int, int, off_t);
-#define mmap mmap64
+# define mmap mmap64
 # define MALLOC_FAILURE_ACTION	__set_ENOMEM ()
 # define USE_DL_PREFIX 1
-#else
+
+#elif defined (__INSIDE_CYGWIN__)
+
 # define __malloc_lock() mallock.acquire ()
 # define __malloc_unlock() mallock.release ()
 extern muto mallock;
+
 #endif
+
 #ifdef __cplusplus
 }
 #endif