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/libc/strfmon.c

@@ -30,6 +30,7 @@
  *
  */
 
+#define __INSIDE_CYGWIN__
 #define _GNU_SOURCE
 #include <sys/cdefs.h>
 __FBSDID("$FreeBSD$");