libstdc++: Fix PSTL tests to run correctly with newer Thread Building Blocks

	* testsuite/lib/libstdc++.exp: Add additional_flags=
	-DTBB_SUPRESS_DEPRECATED_MESSAGES=1 to suppress warnings when
	compiling with a newer Thread Building Blocks.

libstdc++-v3/ChangeLog

+2020-04-20  Thomas Rodgers  <trodgers@redhat.com>
+
+	* testsuite/lib/libstdc++.exp: Add additional_flags=
+	-DTBB_SUPRESS_DEPRECATED_MESSAGES=1 to suppress warnings when
+	compiling with a newer Thread Building Blocks.
+
 2020-04-20  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/24_iterators/istreambuf_iterator/sentinel.cc: New test.

libstdc++-v3/testsuite/lib/libstdc++.exp

@@ -1597,7 +1597,8 @@ proc check_effective_target_tbb-backend { } {
         puts $f "}"
         close $f
 
-        set lines [v3_target_compile $src $exe executable "additional_flags=-std=c++17 additional_flags=-ltbb"]
+        set lines [v3_target_compile $src $exe executable "additional_flags=-std=c++17 additional_flags=-ltbb
+                                                           additional_flags=-DTBB_SUPPRESS_DEPRECATED_MESSAGES=1"]
         file delete $src
 
         if [string match "" $lines] {