test/test_replaygain.sh : Prefer mawk over gawk.

The AWK script used to generate tones for this test fails with gawk
version 4.0.1, but used to work for earlier versions. Use mawk instead
if it exists.

test/test_replaygain.sh

@@ -110,13 +110,22 @@ fi
 
 check_flac
 
+
+if mawk ; then
+	AWK=mawk
+else
+	# Really hope awk is not gawk, because the following AWK script doesn't
+	# work correctly with gawk 4.0.1 but did with earlier versions.
+	AWK=awk
+	fi
+
 # Replay gain tests - Test the rates which have specific filter table entries
 # and verify that harmonics can be processed correctly.
 
 tonegenerator ()
 {
 	# When using GAWK, use --lint=posix to identify non-POSIX awk usages.
-    awk -- '
+    $AWK -- '
     BEGIN {
             samplerate = '$1';