[Ada] PR ada/91268 Do not redefine macros

This should fix PR ada/91268 by only defining macros if not already
done.

2019-09-17  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/

	* adaint.c (_REENTRANT, _THREAD_SAFE): Only define if needed.

From-SVN: r275774

gcc/ada/ChangeLog

 2019-09-17  Arnaud Charlet  <charlet@adacore.com>
 
+	* adaint.c (_REENTRANT, _THREAD_SAFE): Only define if needed.
+
+2019-09-17  Arnaud Charlet  <charlet@adacore.com>
+
 	* libgnat/s-bitfie.ads (Val_Bits, Val_Bytes): Define from
 	Long_Long_Integer'Size.
 

gcc/ada/adaint.c

@@ -35,8 +35,14 @@
    library calls directly. This file contains all other routines.  */
 
 /* Ensure access to errno is thread safe.  */
+
+#ifndef _REENTRANT
 #define _REENTRANT
+#endif
+
+#ifndef _THREAD_SAFE
 #define _THREAD_SAFE
+#endif
 
 /* Use 64 bit Large File API */
 #if defined (__QNX__)