Introduce ATTRIBUTE_UNUSED_RESULT and use it

This introduces the new ATTRIBUTE_UNUSED_RESULT define, and applies it
to gdb_argv::release.

gdb/ChangeLog
2019-03-05  Tom Tromey  <tromey@adacore.com>

	* utils.h (class gdb_argv) <release>: Add
	ATTRIBUTE_UNUSED_RESULT.
	* common/common-defs.h (ATTRIBUTE_UNUSED_RESULT): Define.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2019-03-05  Tom Tromey  <tromey@adacore.com>
+
+	* utils.h (class gdb_argv) <release>: Add
+	ATTRIBUTE_UNUSED_RESULT.
+	* common/common-defs.h (ATTRIBUTE_UNUSED_RESULT): Define.
+
 2019-03-02  Eli Zaretskii  <eliz@gnu.org>
 
 	* xml-syscall.c (xml_list_syscalls_by_group): Drop 'struct' from

gdb/common/common-defs.h

@@ -93,6 +93,12 @@
 #undef ATTRIBUTE_PRINTF
 #define ATTRIBUTE_PRINTF _GL_ATTRIBUTE_FORMAT_PRINTF
 
+#if GCC_VERSION >= 3004
+#define ATTRIBUTE_UNUSED_RESULT __attribute__ ((__warn_unused_result__))
+#else
+#define ATTRIBUTE_UNUSED_RESULT
+#endif
+
 #include "libiberty.h"
 #include "pathmax.h"
 #include "gdb/signals.h"

gdb/utils.h

@@ -188,7 +188,7 @@ public:
   /* Return the underlying array, transferring ownership to the
      caller.  */
 
-  char **release ()
+  ATTRIBUTE_UNUSED_RESULT char **release ()
   {
     char **result = m_argv;
     m_argv = NULL;